5. Physician of Vision
© 2023 Andrew Robinson, CC BY-NC 4.0 https://doi.org/10.11647/OBP.0344.05
His pursuits, diversified as they were, had all originated in the first instance from the study of physic: the eye and the ear led him to the consideration of sound and of light.
Young, ‘Autobiographical sketch’, 1826/27 [139]
The year 1800 marked the beginning of a new century and the most scientifically significant phase of Young’s life, during which he published the work that would change physics and physiology. But it did not launch his career as a physician. Although he was now permitted to practice medicine in London, he was not yet the holder of a Cambridge M.B. degree, could not become an M.D. under Cambridge regulations until five years after that, and would only then be eligible to become a Fellow of the Royal College of Physicians (F.R.C.P.)—these titles were conferred on him in 1803, 1808 and 1809, respectively, considerably later than for most physicians. In the meantime, he would need to begin looking for a suitable position in a hospital as a physician and building up a private practice, since he could expect to inherit no more than a few of his great-uncle’s patients. (The most famous of them, Edmund Burke, had died the previous year, a few months before Brocklesby.)
Perhaps this is why Young decided to sell Brocklesby’s house in Norfolk Street off fashionable Park Lane opposite Hyde Park and to find himself a new address, in 1801. Young left no clue as to his motives in moving house soon after his return to London; but the wish to live more modestly than had Brocklesby, at least until he established himself as a physician, would have been natural and prudent. He probably also wanted to get away, though not far away, from his great-uncle’s shade and the many memories—not all of them pleasant ones—that would have crowded him at 10 Norfolk Street. And perhaps he also sensed that he should locate himself nearer to the emerging centre of the medical profession around Harley Street, an address that within a few decades would become synonymous with top-ranking physicians and surgeons. All these reasons would explain his choice of Welbeck Street, close to Harley Street, where he would live for the next 25 years.
He might well have chosen instead to look for a house in the area around Soho and Leicester Square, where Newton, Hogarth, Reynolds and John Hunter had lived, and where the Hunterian school of anatomy was located in Great Windmill Street. Such an address was certainly prestigious, the atmosphere was intellectual and artistic, and the streets were unquestionably lively, being close to the fashionable Strand, to theatres and to sinful Covent Garden. But maybe the area would have seemed too rakish for an up-and-coming physician, despite Young’s sympathy for Georgian free enterprise (as epitomised by the Hunterian school). He must have preferred somewhere quieter and more sedate in the residential streets that had recently been built north of Oxford Street and south of the New Road, the route laid out in 1756–1757 as the northern boundary of London in the late eighteenth century (now the Marylebone, Euston and Pentonville Roads). In this period, London, having expanded westwards and created the West End earlier in the century, shot northwards—so much so that in 1791 Horace Walpole, the writer and politician, who lived in the West End (in Berkeley Square) to the south of Oxford Street, complained that London’s expansion was killing the sedan-chair trade, ‘for Hercules and Atlas could not carry anybody from one end of this enormous capital to the other’.[140]
These new north London streets and squares—with some substantial mansions built by well-known architects like Robert Adam—were constructed on the fields of the Harley-Cavendish estate, land belonging to Edward Harley, the second earl of Oxford, who had formed a shrewd marriage alliance with the Cavendish family. Starting from the nucleus of Cavendish Square, streets such as Harley Street, Portland Place, Bentinck Street (the latter two named after William Bentinck, the second duke of Portland, who married a Cavendish daughter), and Welbeck Street (named after Welbeck Abbey, the Portland family home), were developed. Among the new residents of the area were the Maxwell family of Cavendish Square, one of whose daughters Young would soon marry; and, a little before Young, Edward Gibbon, author of The History of the Decline and Fall of the Roman Empire, much of which was written at 7 Bentinck Street, just around the corner from Young’s future house, in 1773–1783. The bachelor Gibbon sold his own family estate when his father died so as to buy this desirable new town house in Bentinck Street, and wrote smugly of his good fortune:
I had now attained the solid comforts of life, a convenient well-furnished house, a domestic table, half a dozen chosen servants, my own carriage, and all those decent luxuries whose value is the more sensibly felt the longer they are enjoyed […] To a lover of books the shops and sales in London present irresistible temptations […] By my own choice I passed in town the greatest part of the year.[141]
It is unlikely that Young lived in luxurious style at 48 Welbeck Street, though we cannot be sure about this since he seems never to have described his domestic arrangements in any detail, except for the fact that he kept a horse and servants. But like Gibbon, Young certainly aspired to elegance and high social status, appreciated comfort, and relished London society and metropolitan booksellers, though without any ostentatious display. What mattered most to him was the freedom his considerable wealth gave him to work on the subjects that appealed to his mind. No sooner had he moved into the new house than he was apologising in a letter to Andrew Dalzel, dated 27 June 1801, for the delay in replying to him caused by ‘the confusion of furnishing and entering upon a house.’ There must have been a great many books and some scientific equipment to be transported, too, along with the furniture and pictures, by horse-drawn conveyance from Norfolk Street. He continued:
I am at present employed in some further optical investigations, which, I imagine, will be considered as more important than any of my former attempts, as I think they will establish almost incontrovertibly the undulatory system of light, and extend it to the explanation of an immense variety of phenomena. I have also some prospect of being in a situation which will enable and require me to devote more time to the pursuit of natural philosophy than I should otherwise think consistent with the profession of physic, but the idea is yet only in embryo.[142]
The ‘situation’ Young referred to was the tempting offer of the post of professor of natural philosophy at the Royal Institution, which would keep him extremely busy with (entirely non-medical) lectures in 1802–1803. We shall come to this in the next chapter. As for the ‘further optical investigations’ connected with the wave theory of light (and its comparison with sound), these too fully deserve their own separate chapter, after the Royal Institution lectures—since they did not come to full fruition until the end of 1803, and also because they are fundamentally physics and therefore distinct from Young’s simultaneous physiological work on the human eye and colour vision in 1800–1801, which grew out of his knowledge as a physician.
‘On the mechanism of the eye’, Young’s lengthy and highly detailed paper read to the Royal Society in November 1800, and published in 1801, was called, more than a century later, a ‘masterly monograph’ containing ‘Young’s greatest and most original contributions to science’.[143] Admittedly the comment came from an ophthalmologist and physiologist, a distinguished one, Sir John Parsons, who was perhaps partisan; yet this particular paper has been so widely admired for its experimental ingenuity and the clarity of its deductions that we shall quote from it at length. It was here that Young finally established the process at work in the accommodation of the eye, and also defined and measured astigmatism for the first time.
Bearing in mind the contested nature of the field—the differing ideas of Kepler and Descartes about accommodation, Porterfield’s experiments on couched eyes, Young’s own first paper of 1793, Hunter’s claim to priority, the rumour of plagiarism against Young, the contrary paper by Home published in 1795, Young’s subsequent withdrawal of his thesis about the ‘muscular’ lens and Monro’s scepticism about both Young’s paper and Home’s when Young was a student at Edinburgh, not to mention even more recent evidence from others against the lens as the site of accommodation—Young felt the need for a semi-historical and cautionary preamble. His paper therefore begins:
In the year 1793, I had the honour of laying before the Royal Society, some observations on the faculty by which the eye accommodates itself to the perception of objects at different distances. The opinion which I then entertained, although it had never been placed exactly in the same light, was neither so new, nor so much forgotten, as was supposed by myself, and by most of those with whom I had any intercourse on the subject. Mr Hunter, who had long before formed a similar opinion, was still less aware of having been anticipated in it, and was engaged, at the time of his death, in an investigation of the facts relative to it; an investigation for which, as far as physiology was concerned, he was undoubtedly well qualified. Mr Home, with the assistance of Mr Ramsden, whose recent loss this society cannot but lament, continued the enquiry which Mr Hunter had begun; and the results of his experiments appeared very satisfactorily to confute the hypothesis of the muscularity of the lens. I therefore thought it incumbent on me to take the earliest opportunity of testifying my persuasion of the justice of Mr Home’s conclusions, which I accordingly mentioned in a dissertation published at Göttingen in 1796, and also in an essay presented last year to this society. About three months ago, I was induced to resume the subject, by perusing Dr Porterfield’s paper on the internal motions of the eye; and I have very unexpectedly made some observations, which I think I may venture to say, appear to be finally conclusive in favour of my former opinion, as far as that opinion attributed to the lens a power of changing its figure. At the same time, I must remark, that every person who has been engaged in experiments of this nature, will be aware of the extreme delicacy and precaution requisite, both in conducting them, and in drawing inferences from them; and will also readily allow, that no apology is necessary for the fallacies which have misled many others, as well as myself, in the application of those experiments to optical and physiological determinations.[144]
To understand the paper itself, we need first to review the basic structure of the human eye, which was understood in Young’s day, except for the function of the ciliary body. As shown in Figure 5.1, the front of the eye has an outer transparent cover, the cornea, which merges into the non-transparent white of the eye, the outer covering of the eyeball, known as the sclera. The cornea encloses a space filled with a fluid called the aqueous humour. Behind this is the iris diaphragm, which controls the size of the pupil, as in a camera. And behind the iris is the crystalline lens surrounded by the ciliary muscles, the inner space of the eye filled with a second fluid known as vitreous humour, and the light-sensitive inner coating of the inside of the eye, the retina, which is connected to the optic nerve and through it to the brain.
![Anatomical diagram showing the general components of the human eye: the sclera, zonules, iris, aqueous humour, crystalline lens, pupil, cornea, ciliary muscle, vitreous humour, retina, and optic nerve.](image/image-05.jpg)
Fig. 5.1 Cross-section of the human eye. The ciliary muscles and zonules were unknown to Young, who assumed the lens was muscular.
Rays of light from a point on an object, travelling through the air-cornea interface into the aqueous humour, are bent by refraction (compare the apparent bending of a pencil dipped in a cup of water). Passing through the pupil, the rays are then further bent by refraction in the crystalline lens. About two thirds of the eye’s bending takes place at the air-cornea interface, the rest in the lens. If the eye is correctly focused for the distance of the object from it, the rays collect at a point on the retina, and a sharp image of the object is formed by the brain. But if the eye is not correctly focused, then the image forms either in front of the retina or behind it, depending on whether the rays are bent too much or too little, and the brain perceives an out-of-focus image of the object. In order to correct the focus, as mentioned in Chapter 2, ‘Fellow of the Royal Society’, the eye naturally accommodates so as to be able to focus light rays on the retina from objects at different distances.
If Young was to grasp this mysterious process of accommodation, he needed to be able to monitor and measure it as accurately as possible. Over what range of distance from the eye could any particular eye accommodate and produce focused images? He therefore developed an optometer. Its principle was not new, having been suggested by Christoph Scheiner in 1619, nor was its implementation, which was carried out by Porterfield, but the accuracy and practicality of the instrument were greatly improved by Young.
Scheiner’s observation was as follows (as translated into clearer language in the nineteenth century by Hermann Helmholtz). Make two pin-holes in a card at a small distance apart less than the diameter of the pupil of the eye. Look through them with one eye as close to the pinholes as possible, keeping the other eye closed, at a small, clearly delineated object, such as a needle held in front of a bright window. Keep the object vertical, at right angles to the line joining the pinholes. Focus the eye on the object so that it is sharp. If you now shift the focus of your eye to something else, in front of the object or behind it, the object will appear double. Focus again on the object and the double images cross, coincide and appear single again.
Porterfield realised that this fact could be used to measure the near point of an eye—that is, the nearest to the eye an object can be brought while still remaining focusable and sharp (not double)—and the far point—the furthest distance away at which the object will remain sharp; and that these two points would give the range of the eye’s accommodation. Instead of pinholes, Porterfield used narrow vertical slits to increase the visibility of the object, and a movable vertical slit in a lamp lit by a candle as the object to be viewed. When the eye saw the illuminated slit as a single line, rather than a double line, the eye was focused. Young’s further development of the optometer was twofold: he incorporated a graduated scale giving the distance between the two slits (and the eye) and the single illuminated slit, and he added a convex lens close to the two slits. The purpose of this lens was to overcome the fact that the far point of a normal eye is at an infinite distance from the two slits, which clearly cannot be measured by the scale. The effect of the lens meant that all the distances measured by the optometer (not merely the far point), including the near point, had to be adjusted to give the ‘true’ focal distance—from which could be calculated the power of a spectacle lens required to correct short and long sight. Young made his optometers, which varied in size, out of both card and ivory; one still exists at the Royal Institution.
The near point of his own eye, after he had made the adjustment for the convex lens, turned out to be eight inches; and he took this to be normal. Today, a near point of ten inches is considered normal. This means that Young was somewhat short-sighted; the more short-sighted a person is, the nearer to the eye is his or her near point. In due course Young must have realised his myopia, because in his autobiographical sketch he writes: ‘He felt some inconvenience in society from being a little short sighted, and he used to attribute in part to this circumstance the mistakes which he sometimes made respecting the impression produced by what he said or did, on the feelings of others.’[145] (It seems possible that the frequent cases of mistaken identity in the dramatic plots of plays and operas of Young’s age were more convincing to audiences then than they are now, because many people were short-sighted and did not wear spectacles.)
Young also experimented with the optometer to determine his far point. In the course of this process, he noted:
My eye, in a state of relaxation, collects to a focus on the retina, those rays which diverge vertically from an object at the distance of ten inches from the cornea, and the rays which diverge horizontally from an object at seven inches distance. For, if I hold the plane of the optometer vertically, the images of the line appear to cross at ten inches; if horizontally, at seven. […] I have never experienced any inconvenience from this imperfection, nor did I ever discover it till I made these experiments; and I believe I can examine minute objects with as much accuracy as most of those whose eyes are differently formed.[146]
Although Young did not name this condition (it was named three decades or so later by William Whewell, the polymathic master of Trinity College, Cambridge, in a suggestion to the seriously astigmatic astronomer Sir George Biddell Airy), Young’s comment is the first scientific recognition of astigmatism. Its name derives from the Greek for ‘not at a point (stigma)’. In an eye with astigmatism, the rays from a vertical line are focused differently to the rays from a horizontal line, and so the various rays do not collect at points in the same focal plane, with the result that the image is blurred. An optician tests for astigmatism by showing the patient a card with a series of radiating black lines. If astigmatism is present, one particular line will seem sharp, and the line at right angles to it will appear fuzzy.
When Young mentioned this experiment to a scientific instrument maker, William Cary, Cary told him that he had frequently observed the condition, and ‘that many persons were obliged to hold a concave glass obliquely, in order to see with distinctness: counterbalancing, by the inclination of the glass, the too great refractive power of the eye in the direction of that inclination’.[147] From this, Young concluded that astigmatism was due to the crystalline lens in astigmatic eyes being at a slightly oblique angle to the vertical axis, and suggested that it could be compensated for by tilting a spectacle lens or the eyeglass of a telescope. While this is true, he was incorrect in dismissing the role of the cornea in astigmatism; today we know that corneal imperfections—a lack of symmetry in the curvature of the cornea so that different parts of it refract rays to slightly differing extents—are actually a much more common cause of astigmatism than misalignment of the crystalline lens.
Having found a relatively convenient way to measure the eye’s focal distance with his optometer, Young now devised a method to measure the dimensions of his eye: its diameter and its length from back to front. His technique here, and in most of the experiments in his paper, was not for the clumsy or the faint-hearted, and belongs in a long and honourable tradition of scientists experimenting on themselves. One must imagine Young, all alone in his house (still at this time in Norfolk Street) except perhaps for a servant, performing risky operations on his eyes, surrounded by candles, mirrors, lenses, microscopes, optometers and other homemade apparatus. ‘For measuring the diameters, I fix a small key on each point of a pair of compasses; and I can venture to bring the rings [of the keys] into immediate contact with the sclerotica [sclera]. The transverse diameter is externally ninety-eight hundredths of an inch.’[148] To measure the distance from the back of the retina to the front of the cornea was rather more tricky (and painful!). He turned his eye inwards as far as it would go. Then he pushed the ring of one key in at the back of the eye and pressed on the back of the eyeball to produce the sensation of a bright spot on his retina—indicating that the key was almost touching the retina—in the centre of his field of vision, coinciding with the direction of the eye’s optical axis. ‘With an eye less prominent, this method might not have succeeded.’[149] Then, by looking into a mirror, he brought the second ring on the pair of compasses in contact with the cornea at the front of the eye. The distance between the back of the retina and the cornea turned out to be ninety-one hundredths of an inch, which was slightly less than the transverse diameter. Hence, the eyeball was not exactly spherical. From these figures, he could calculate the radius of curvature of the cornea. His measurements and calculations match extraordinarily well with modern measurements.
Then we come to the process of accommodation. Young wished to test four hypotheses for what happens during accommodation:
- The curvature of the cornea changes.
- The length of the eyeball changes.
- Both changes occur at the same time.
- The shape of the crystalline lens changes.
To test hypothesis one, he devised a series of experiments, of which we shall describe only two. The first consisted of a very careful examination with a graduated microscope of the reflection of a candle flame in the cornea of the eye of an assistant. Young’s idea was to check whether the size of reflection varied as his assistant focused his eye on objects at different distances. It should have varied if the curvature of the cornea changed during accommodation but stayed the same if the curvature remained the same. In his own words:
I placed two candles so as to exhibit images in a vertical position in the eye of Mr König, who had the goodness to assist me; and, having brought them into the field of the microscope, where they occupied 35 of the small divisions, I desired him to fix his eye on objects at different distances in the same direction: but I could not perceive the least variation in the distance of the images.[150]
The second, and the most crucial of all the experiments, involved immersing the eye in water. As underwater swimmers are aware, the unaided eye cannot focus sharply in water. The explanation for this is that light passing through the water-cornea interface into the aqueous humour is no longer refracted because the aqueous humour, optically speaking, is very nearly equivalent to water. (Recall that it is the air-cornea interface that causes about two-thirds of the refraction in the eye.) Young reasoned that if, in front of an eye in water, he were to add a lens with a refractive power equivalent to the eliminated air-cornea interface, the eye should be able to focus again. If, in addition, the immersed eye with the extra lens could still accommodate, then the process of accommodation could not involve the cornea.
Here is how he describes the experiment:
I take out of a small botanical microscope, a double convex lens, of eight-tenths radius and focal distance, fixed in a socket one-fifth of an inch in depth; securing its edges with wax, I drop into it a little water, nearly cold, till it is three fourths full, and then apply it to my eye, so that the cornea enters halfway into the socket, and is everywhere in contact with the water. My eye immediately becomes presbyopic [i.e., long-sighted, because the loss in refraction at the cornea means that the image forms behind the retina], and the refractive power of the lens, which is reduced by the water […] is not sufficient to supply the place of the cornea, rendered inefficacious by the interventions of the water; but the addition of another lens of five inches and a half focus, restores my eye to its natural state and somewhat more. I then apply the optometer, and I find the same inequality in the horizontal and vertical refractions as without the water [demonstrating that Young’s astigmatism was not corneal in origin]; and I have, in both directions, a power of accommodation equivalent to a focal length of four inches, as before […] After this it is almost necessary to apologise for having stated the former experiments; but, in so delicate a subject, we cannot have too great a variety of concurring evidence.[151]
With his first hypothesis now abandoned, Young designed experiments to test hypothesis two: that the eyeball changed its length during accommodation, like a camera lens adjusting for focus. His method was to fix the length of the eyeball mechanically, so that it could not expand or contract, and then try to focus his eye on objects at different distances. If, under these conditions, his eye could still accommodate, then accommodation could not be due to the change in length of the eyeball.
The most important of this second group of experiments was described by Young, somewhat unnervingly, as follows:
[A] much more delicate [test], was the application of the ring of a key at the external angle, when the eye was turned as much inwards as possible, and confined at the same time by a strong oval iron ring, pressed against it at the internal angle. The key was forced in as far as the sensibility of the integuments would admit, and was wedged, by a moderate pressure, between the eye and the bone. In this situation the phantom [the bright spot on the retina] caused by the pressure extended within the field of perfect vision, and was very accurately defined.[152]
With the eye held in this state, he argued, the phantom was a highly sensitive indicator of the length of the eyeball. The slightest increase or decrease in length would increase or decrease the pressure on the retina, and alter the size and shape of the phantom. (His paper includes his drawings of this illusion under various conditions.) ‘But no such circumstance took place; the power of accommodation was as extensive as ever; and there was no perceptible change either in the size or in the figure of the oval spot.’[153]
Hypothesis three—that accommodation was due to a mixture of change in corneal curvature and eyeball length—was now obviously ruled out. By process of elimination, it appeared that hypothesis four—change in the shape of the crystalline lens—was the most likely to be true. Young set about trying to find experimental evidence for it.
Home, with the help of Ramsden, had claimed in 1795 that a man they had examined named Benjamin Clerk, whose eye had been couched because he had a cataract, afterwards retained the power of accommodation. Clerk, a seafarer, had gone missing and was unavailable to Young in 1800, but an optician friend of Young’s, a Mr Ware, introduced him to five of his patients who he thought might make suitable subjects. Ware had originally been convinced by Home’s paper but subsequently had noticed that all of his patients who had been couched derived ‘obvious advantage’[154] from using two kinds of spectacles—one for close-up work such as reading, the other for seeing at a distance. This strongly implied they had a deficiency in their power of accommodation. (Bifocal spectacles were invented to deal with this problem in older people, whose eyes generally lose some power of accommodation in their forties.)
None of the five subjects was perfect for Young’s purposes, as he made clear in his report on each of them. But after very carefully testing them all with his optometers, in the presence of their optician Mr Ware, he came to a firm conclusion: ‘the universal result is, contrary to the expectation with which I entered on the inquiry, that in an eye deprived of the crystalline lens, the actual focal distance is totally unchangeable.’[155] This would be fully confirmed only when Benjamin Clerk was located again some years later and tested in the presence of Home, Young and two others. Their joint examination was extremely painstaking—not least because the reputations of the physicians as scientists were at stake—and Young noted with satisfaction in his Introduction to Medical Literature, published in 1813, that ‘the imperfect eye, from which the crystalline lens had been extracted, possessed no power whatever of altering its focus, while the same tests exhibited a very considerable change in the focal distance of the perfect eye’.[156]
In his pioneering 1800 paper, he had thus established, at least provisionally, the process of accommodation of the eye; but he still needed to put forward a mechanism for the process. The crystalline lens must change shape, he had now shown, but how did it do this? In 1793, he had maintained that the lens was muscular. In 1800, he was less certain, after re-examining the anatomy of the eye: ‘Now, whether we call the lens a muscle or not, it seems demonstrable, that such a change of figure [shape] takes place as can be produced by no external cause; and we may at least illustrate it by a comparison with the usual action of muscular fibres.’[157] Here Young was less perceptive, and it would be left to others, including Helmholtz, later in the century, to identify the thin threads of robust material, called zonules, that hold the non-muscular crystalline lens in place and are attached to the ciliary body housing the ciliary muscle.
However, Young was careful not to include the muscular lens hypothesis in his final paragraph summing up what he saw as the definite results achieved in his paper ‘On the mechanism of the eye’. This paragraph has often been quoted by physiologists for its clarity and concision:
First, the determination of the refractive power of a variable medium, and its application to the constitution of the crystalline lens. Secondly, the construction of an instrument for ascertaining, upon inspection, the exact focal distance of every eye, and the remedy for its imperfections. Thirdly, to show the accurate adjustment of every part of the eye, for seeing with distinctness the greatest possible extent of objects at the same instant. Fourthly, to measure the collective dispersion of coloured rays in the eye. Fifthly, by immerging [immersing] the eye in water, to demonstrate that its accommodation does not depend on any change in the curvature of the cornea. Sixthly, by confining the eye at the extremities of its axis, to prove that no material alteration of its length can take place. Seventhly, to examine what inference can be drawn from the experiments hitherto made on persons deprived of the lens; to pursue the inquiry on the principles suggested by Dr Porterfield; and to confirm his opinion of the utter inability of such persons to change the refractive state of the organ. Eighthly, to deduce, from the aberration of the lateral rays [astigmatism], a decisive argument in favour of a change in the figure of the crystalline; to ascertain, from the quantity of this aberration, the form into which the lens appears to be thrown in my own eye, and the mode by which the change must be produced in that of every other person.[158]
Young’s other major contribution to understanding the eye came in a second lecture, ‘On the theory of light and colours’, given to the Royal Society almost exactly a year later (after he had moved from Norfolk to Welbeck Street), and published in 1802. This is where he put forward his theory of three-colour vision. But its presentation could hardly be more different from the detailed experimentation and calculation documented in the first lecture. His far-sighted idea was more like an intuition, an aperçu, than a developed theory—and it would take a century and a half before it was verified experimentally. ‘Surely the most prescient work in all of psychophysics’[159], a physicist called it in 1989 (Walter Moore, in his scientific biography of Erwin Schrödinger, another physicist with wide interests, including colour vision). Young himself thought so comparatively little of the idea that he did not even mention his three-colour theory in his list of publications at the end of his autobiographical sketch.
In the seventeenth century, Newton had split white light into the colours of a spectrum with a prism and reconstituted the spectrum into white light with a second prism; he had also used a second prism to show that the individual colours of the spectrum could not be further split. In 1672, Newton introduced the term ‘primary colours’, and pondered how many such colours there were, favouring seven, and how discrete colours might relate to a clearly continuous spectrum, in his Opticks, published in 1704. During the eighteenth century, the concept of primary colours became generally accepted, but they were reduced to three in number, usually red, yellow and blue. Yet there was no understanding of how these primary colours could create the great variety of hues—more than 150 of them—distinguishable by the eye, mainly because the relationship between colour and wavelength was not appreciated, in the absence of acceptance of a wave theory of light.
Young’s brilliant insight, stimulated by his embrace of the undulatory/wave theory during 1801, was to imagine how the retina might actually detect the sensation of colour. He wrote:
Now, as it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance, to the three principal colours, red, yellow, and blue, of which the undulations are related in magnitude nearly as the numbers 8, 7, and 6; and that each of the particles is capable of being put in motion less or more forcibly by undulations differing less or more from a perfect unison; for instance, the undulations of green light being nearly in the ratio of 6.5, will affect equally the particles in unison with yellow and blue, and produce the same effect as a light composed of those two species; and each sensitive filament of the nerve may consist of three portions, one for each principal colour.[160]
In other words, the brain would perceive red light, with the longest wavelength, as red because it would stimulate (be in ‘perfect unison’ with) only one type of receptor (‘particle’) in the retina; ditto for yellow light, with a shorter wavelength, which would stimulate only a second type of receptor; and for blue light, of an even shorter wavelength, which would stimulate only a third type of receptor. Light of an intermediate wavelength, such as green, would stimulate both the yellow and the blue receptors, though less strongly than yellow and blue light; and the mixture of the two sensations would be perceived as green in the brain. Young had ‘proposed a theory of colour vision which, for the first time, suggested the brain may not only receive information, but [that] it processes and integrates the information it receives’, the physiologist J. Z. Young observed long after.[161]
The following year, 1802, as a result of experiments on the colour spectrum by the physicist and chemist William Hyde Wollaston, Young changed his choice of ‘principal colours’ to which the retina was sympathetic from red, yellow and blue to red, green and violet. This is the work that lay fallow in the Philosophical Transactions of the Royal Society until it was rediscovered by an excited Helmholtz in the 1850s and developed into the Young-Helmholtz theory of colour vision, which was soon confirmed and extended by the experiments of James Clerk Maxwell with spinning tops painted with sections of different colour (an idea which Young had also written about). Yet it took until 1959 before scientists made ‘the definitive experiments that finally proved Young’s idea that colour must depend on a retinal mosaic of three kinds of detectors,’[162] commented David Hubel, a twentieth-century authority on visual neuroscience. The experiments were the work of two groups in the United States—those of George Wald and Paul Brown at Harvard University, and of William Marks, William Dobelle and Edward MacNichol at Johns Hopkins University—who examined the cones in the retina and their ability to absorb light of different wavelengths and discovered just three cone types, as speculated by Young in 1801.
Wald went on to explain colour blindness in terms of a reduced or absent receptivity in one or more of the three cone types. Here again Young had led the way. He was interested in the colour blindness of one of his contemporaries, the chemist John Dalton, who in 1798 stirred great interest by describing how red, orange, yellow and green were akin to him, but how he could distinguish blue and purple. Dalton was convinced that the cause was his vitreous humour being tinged blue (and therefore absorbing red light before it reached the retina). But Young did not agree with Dalton’s notion, remarking in his published Royal Institution lectures that ‘this has never been observed by anatomists, and it is much simpler to suppose the absence or paralysis of those fibres of the retina which are calculated to perceive red; this supposition explains all the phenomena’.[163] Dalton’s vitreous humour was tested after his death in 1844 (at his written request) and found to be colourless, supporting Young; but when the retina from one of his preserved eyes was examined in the 1990s, the evidence was less supportive of Young, since it lacked the photo-pigment sensitive to light of middle wavelength rather than the longer-wavelength red light.
The moment has now arrived to leave Young’s contributions to physiology and turn to his lectures on physics and related subjects at the Royal Institution in 1802–1803. The polymath was about to face the public in London for the first time. The encounter would prove to be a disturbing one, both for Young and for his listeners.
Notes and References
Note that the precise wording of the quotations from Young’s letters, the originals of which were available to George Peacock and Alex Wood but have since disappeared, sometimes differs in their two biographies; in each case, I have chosen what appears to me to be the most reliable version.
[139] Quoted in Hilts: 253.
[140] Quoted in Porter, London: 123.
[141] Ibid: 141.
[142] Letter to Dalzel (27 June 1801) in Dalzel: 206.
[143] Quoted in Wood: 104. Parsons made the comment in 1930.
[144] Young, Miscellaneous Works, vol. 1: 12–13.
[145] Quoted in Hilts: 254.
[146] Young, Miscellaneous Works, vol. 1: 26.
[147] Ibid: 26.
[148] Ibid: 25.
[149] Ibid: 25.
[150] Ibid: 39.
[151] Ibid: 41.
[152] Ibid: 42.
[153] Ibid: 42–43.
[154] Ibid: 46.
[155] Ibid: 46.
[156] Young, An Introduction to Medical Literature: 99.
[157] Young, Miscellaneous Works, vol. 1: 51.
[158] Ibid: 60–61.
[159] Moore, Schrödinger : 122.
[160] Miscellaneous Works, vol. 1: 147.
[161] Kline: 3. Kline notes that this idea was pointed out to him by J. Z. Young.
[162] Hubel: 168.
[163] ‘Catalogue—physical optics’ in Young, Natural Philosophy, vol. 3: 315. See Wade, A Natural History of Vision: 136–42, for a brief history of colour blindness, which mentions the tests on Dalton’s eyes.