Part 11 (1/2)

he showed that sunlight is made up of light varying in tint from red, through orange, yellow, green, and blue, to violet, and that by recombining all these kinds of light, or certain of them selected in an indefinite number of ways, white light could be produced.

Subsequently Sir Wm. Herschel showed that rays less refrangible than the red were to be found among the solar radiation; and other rays more refrangible than the violet, but, like the ultra-red rays, incapable of exciting vision, were found by Ritter and Wollaston. In speaking of Newton's experiments, in his thirty-seventh lecture, Young says:--

It is certain that the perfect sensations of yellow and of blue are produced respectively by mixtures of red and green and of green and violet light, and there is reason to suspect that those sensations are always compounded of the separate sensations combined; at least, this supposition simplifies the theory of colours. It may, therefore, be adopted with advantage, until it be found inconsistent with any of the phenomena; and we may consider white light as composed of a mixture of red, green, and violet only, ... with respect to the quant.i.ty or intensity of the sensations produced.

It should be noticed that, in the above quotation, Young speaks only of the sensations produced. Objectively considered, sunlight consists of an infinite number of differently coloured lights comprising nearly all the shades from one end of the spectrum to the other, though white light may have a much simpler const.i.tution, and may, for example, consist simply of a mixture of h.o.m.ogeneous red, green, and violet lights, or of h.o.m.ogeneous yellow and blue lights, properly selected.

But considered subjectively, Young implies that the eye perceives three, and only three, distinct colour-sensations, corresponding to pure red, green, and violet; that when these three sensations are excited in a certain proportion, the complex sensation is that of white light; but if the relative intensities of the separate sensations differ from these ratios, the perception is that of some colour. To exhibit the effects of mixing light of different colours, Young painted differently coloured sectors on circles of cardboard, and then made the discs rotate rapidly about their centres, when the effect was the same as though the lights emitted by the sectors were mixed in proportion to the breadth of the sectors. This contrivance had been previously employed by Newton, and will be again referred to in connection with another memoir. The results of these experiments were embodied by Young in a diagram of colour, consisting of an equilateral triangle, in which the colours red, green, and violet, corresponding to the simple sensations, were placed at the angles, while those produced by mixing the primary colours in any proportions, were to be found within the triangle or along its sides; the rule being that the colour formed by the admixture of the primary colours in any proportions, was to be found at the centre of gravity of three heavy particles placed at the angular points of the triangle, with their ma.s.ses proportioned to the corresponding amounts of light. Thus the colours produced by the admixture of red and green only, in different proportions, were placed along one side of the triangle, these colours corresponding to various tints of scarlet, orange, yellow, and yellowish green; another side contained the mixtures of green and violet representing the various shades of bluish green and blue; and the third side comprised the admixtures of red and violet const.i.tuting crimsons and purples. The interior of the triangle contained the colours corresponding to the mixture of all three primary sensations, the centre being neutral grey, which is a pure white faintly illuminated. If white light of a certain degree of intensity fall on white paper, the paper appears white, but if a stronger light fall on another portion of the same sheet, that which is less strongly illuminated appears grey by contrast. Shadows thrown on white paper may possess any degree of intensity, corresponding to varying shades of neutral grey, up to absolute blackness, which corresponds to a total absence of light. Thus considered, chromatically black and white are the same, differing only in the amount of light they reflect. A piece of white paper in moonlight is darker than black cloth in full sunlight.

It must be remembered that Young's diagram of colours corresponds to the admixture of coloured lights, not of colouring materials or pigments. The admixture of blue and yellow lights in proper proportions may make white or pink, but never green. The admixture of blue and yellow pigments makes a green, because the blue absorbs nearly all the light except green, blue, and a little violet, while the yellow absorbs all except orange, yellow, and green. The green light is the only light common to the two, and therefore the only light which escapes absorption when the pigments are mixed. Another point already noticed must also be carefully borne in mind. Young was quite aware that, physically, there are an infinite number of different kinds of light differing continuously in wave-length from the ultra-red to the ultra-violet, though colour can hardly be regarded as an attribute of the light considered objectively. The question of colour is essentially one of perception--a physiological, not a physical, question--and it is only in this sense that Young maintained the doctrine of three primary colours. In his paper on the production of colours, read before the Royal Society on July 1, 1802, he speaks of ”the proportions of the sympathetic fibres of the retina,” corresponding to these primary colour-sensations. According to this doctrine, white light would always be produced when the three sensations were affected in certain proportions, whether the exciting cause were simply two kinds of h.o.m.ogeneous light, corresponding to two pure tones in music, or an infinite number of different kinds, as in sunlight; and a particular yellow sensation might be excited by h.o.m.ogeneous yellow light from one part of the spectrum, or by an infinite number of rays of different wave-lengths, corresponding to various shades of red, orange, yellow, and green. Subjectively, the colours would be the same; objectively, the light producing them would differ exceedingly.

But Young's greatest service to science was his application of the principle of interference--of which he had already made good use in the theory of sound--to the phenomena of light. The results of these researches were presented to the Royal Society, and two of the papers were selected as Bakerian lectures in 1801 and 1803 respectively.

Unfavourable criticisms of these papers, which appeared in the _Edinburgh Review_, and were said to have been written by Mr.

(afterwards Lord) Brougham, seem to have caused their contents to be neglected by English men of science for many years; and it was to Arago and Fresnel that we are indebted for recalling public attention to them. The undulatory theory of light, which maintains that light consists of waves transmitted through an _ether_, which pervades all s.p.a.ce and all matter, owes its origin to Hooke and Huyghens. Huyghens showed that this theory explained, in a very beautiful manner, the laws of reflection and of refraction, if it be allowed that light travels more slowly the denser the medium. According to the celebrated principle of Huyghens, every point in the front of a wave at any instant becomes a centre of disturbance, from which a secondary wave is propagated. The fronts of these secondary waves all lie on a surface, which becomes the new surface of the primary wave. When light enters a denser medium obliquely, the secondary waves which are propagated within the denser medium extend to a less distance than those propagated in the rarer medium, and thus the front of the primary wave becomes bent at the point where it meets the common surface. Huyghens explained, not only the laws of ordinary refraction in this manner, but, by supposing the secondary waves to form spheroids instead of spheres, he obtained the laws of refraction of the extraordinary ray in Iceland-spar. He did not, however, succeed in explaining why light should not diverge laterally instead of proceeding in straight lines. Newton supported the theory that light consists of particles or corpuscles projected in straight lines from the luminous body, and sometimes transmitted, sometimes reflected, when incident on a transparent medium of different density. To account for the particle being sometimes transmitted and sometimes reflected, Newton had recourse to the hypothesis of ”fits of easy transmission and of easy reflection,” and, to account for the fits themselves, he supposed the existence of an ether, the vibrations of which affected the particles. The laws of reflection were readily explained, being the same as for a perfectly elastic ball; the laws of refraction admitted of very simple explanation, by supposing that the particles of the denser medium exert a greater attraction on the particles of light than those of the rarer medium, but that this attraction acts only through very short distances, so that when the light-corpuscle is at a sensible distance from the surface, it is attracted equally all round, and moves as though there were no force acting upon it. As a consequence of this hypothesis, it follows that the velocity of light must be greater the denser the medium, while the undulatory theory leads to precisely the opposite result. When Foucault directly measured the velocity of light both in air and water, and found it less in the denser medium, the result was fatal to the corpuscular theory.

Dr. Young called attention to another crucial test between the two theories. When a piece of plate-gla.s.s is pressed against a slightly convex lens, or a watch-gla.s.s, a series of coloured rings is formed by reflected light, with a black spot in the centre. This was accounted for by Newton by supposing that the light which was reflected in any ring was in a fit of easy transmission (from gla.s.s to air) when it reached the first surface of the film of air, and in a fit of easy reflection when it reached the second surface. By measuring the thickness of a film of air corresponding to the first ring of any particular colour, the length of path corresponding to the interval between two fits for that particular kind of light could be determined. When water instead of air is placed between the gla.s.ses, according to the corpuscular theory the rings should expand; but according to the undulatory theory they should contract; for the wave-length corresponds to the distance between successive fits of the same kind on the corpuscular hypothesis. On trying the experiment, the rings were seen to contract. This result seemed to favour the undulatory theory; but the objection urged by Newton that rays of light do not bend round obstacles, like waves of sound, still held its ground. This objection Young completely demolished by his principle of interference. He showed that when light pa.s.ses through an aperture in a screen, whatever the shape of the aperture, provided its width is large in comparison with the length of a wave of light (one fifty-thousandth of an inch), no sensible amount of light will reach any point not directly in front of the aperture; for if any point be taken to the right or left, the disturbances reaching that point from different points of the aperture will neutralize one another by interference, and thus no light will be appreciable. When the breadth of the aperture is only a small multiple of a wave-length, then there will be some points outside the direct beam at which the disturbances from different points of the aperture will not completely destroy one another, and others at which they will destroy one another; and these points will be different for light of different wave-lengths. In this way Young not only explained the rectilinear propagation of light, but accounted for the coloured bands formed when light diverges from a point through a very narrow aperture. In a similar way he accounted for the hyperbolic bands of colour observed by Grimaldi within the shadow of a square near its corners. With a strip of card one-thirtieth of an inch in width, Young obtained bands of colour within the shadow which completely disappeared when the light was cut off from either side of the strip of card, showing that they were produced by interference of the two portions of light which had pa.s.sed, one to the right, the other to the left, of the strip of card.

Professor Stokes has succeeded in showing a bright spot at the centre of the shadow of a circular disc of the size of a sovereign. The narrow bands of colour formed near the edge of the shadow of any object, which Newton supposed to be due to the ”inflection” of the light by the attraction of the object, Young showed to be independent of the material or thickness of the edge, and completely accounted for them by the principle of interference. Newton's rings were explained with equal facility. They were due to the interference of light reflected from the first and second surfaces of the film of air or water between the gla.s.ses. The black spot at the centre of the reflected rings was due to the difference between reflection taking place from the surface of a denser or a rarer medium, half an undulation being lost when the reflection takes place in gla.s.s at the surface of air. If a little grease or water be placed between two pieces of gla.s.s which are nearly in contact, but the s.p.a.ce between be not filled with the water or grease, but contain air in some parts, and water or grease in others, a series of rings will be seen by transmitted light, which have been called ”the colours of mixed plates.” Young showed that these colours could be accounted for by interference between the light that had pa.s.sed through the air and that which had pa.s.sed through the water, and explained the fact that, to obtain the same colour, the distance between the plates must be much greater than in the case of Newton's rings.

The bands of colour produced by the interference of light proceeding from a point and pa.s.sing on each side of a narrow strip of card, have already been referred to. The bands are broader the narrower the strip of card. A fine hair gives very broad bands. When a number of hairs cross one another in all directions, these bands form circular rings of colour. If the width of the hairs be very variable, the rings formed will be of different sizes and overlapping one another, no distinct series will be visible; but when the hairs are of nearly the same diameter, a series of well-defined circles of colour, resembling Newton's rings, will be seen, and if the diameter of a particular ring be measured, the breadth of the hairs can be inferred. Young practically employed this method for measuring the diameter of the fibres of different qualities of wool in order to determine their commercial value. The instrument employed he called the _eriometer_.

It consisted of a plate of bra.s.s pierced with a round hole about one-thirtieth of an inch in diameter in the centre, and around this a small circle, about one-third of an inch in diameter, of very fine holes. The plate was placed in front of a lamp, and the specimen of wool was held on wires at such a distance in front of the bra.s.s plate that the first green ring appeared to coincide with the circle of small holes. The eye was placed behind the lock of wool, and the distance to which the wool had to be removed in front of the bra.s.s plate in order that the first green ring might exactly coincide with the small circle of fine holes, was proportional to the breadth of the fibres. The same effect is produced if fine particles, such as lycopodium powder, or blood-corpuscles, scattered on a piece of gla.s.s, be subst.i.tuted for the lock of wool, and Young employed the instrument in order to determine the diameter of blood-corpuscles. He determined the constant of his apparatus by comparison with some of Dr.

Wollaston's micrometric observations. The coloured halos sometimes seen around the sun Young referred to the existence of small drops of water of nearly uniform diameter, and calculated the necessary diameter for halos of different angular magnitudes.

The same principle of interference afforded explanation of the colours of striated surfaces, such as mother-of-pearl, which vary with the direction in which they are seen. Viewed at one angle light of a particular colour reflected from different ridges will be in a condition to interfere, and this colour will be absent from the reflected light. At a different inclination, the light reaching the eye from all the ridges (within a certain angle) will be in precisely the same phase, and only then will light of that colour be reflected in its full intensity. With a micrometer scale engraved on gla.s.s by Coventry, and containing five hundred lines to the inch, Young obtained interference spectra. Modern gratings, with several thousand lines to the inch, afford the purest spectra that can be obtained, and enable the wave-length of any particular kind of light to be measured with the greatest accuracy.

Young's dislike of mathematical a.n.a.lysis prevented him from applying exact calculation to the interference phenomena which he observed, such as subsequently enabled Fresnel to overcome the prejudice of the French Academy and to establish the principle on an incontrovertible footing. Young's papers attracted very little attention, and Fresnel made for himself many of Young's earlier discoveries, but at once gave Young the full credit of the work when his priority was pointed out.

The phenomena of polarization, however, still remained unexplained.

Both Young and Fresnel had regarded the vibrations of light as similar to those of sound, and taking place in the direction in which the wave is propagated. The fact that light which had pa.s.sed through a crystal of Iceland-spar, was differently affected by a second crystal, according to the direction of that crystal with respect to the former, showed that light which had been so transmitted was not like common light, symmetrical in all azimuths, but had acquired sides or poles.

Such want of symmetry could not be accounted for on the hypothesis that the vibrations of light took place at right angles to the wave-front, that is, in the direction of propagation of the light. The polarization of light by reflection was discovered by Malus, in 1809.

In a letter written to Arago, in 1817, Young hinted at the possibility of the existence of a component vibration at right angles to the direction of propagation, in light which had pa.s.sed through Iceland-spar. In the following year Fresnel arrived independently at the hypothesis of transverse vibrations, not as const.i.tuting a small component of polarized light, but as representing completely the mode of vibration of all light, and in the hands of Fresnel this hypothesis of transverse vibrations led to a theory of polarization and double refraction both in uniaxal and biaxal crystals which, though it can hardly be regarded as complete from a mechanical point of view, is nevertheless one of the most beautiful and successful applications of mathematics to physics that has ever been made. To Young, however, belongs the credit of suggesting that the spheroidal form of the waves in Iceland-spar might be accounted for by supposing the elasticity different in the direction of the optic axis and at right angles to that direction; and he ill.u.s.trated his view by reference to certain experiments of Chladni, in which it had been shown that the velocity of sound in the wood of the Scotch fir is different along, and perpendicular to, the fibre in the ratio of 5 to 4. Young was also the first to explain the colours exhibited by thin plates of crystals in polarized light, discovered by Arago in 1811, by the interference of the ordinary and extraordinary rays, and Fresnel afterwards completed Young's explanation in 1822.

It is for his contributions to the undulatory theory of light that Young will be most honourably remembered. Hooke, in 1664, referred to light as a ”quick, short, vibrating motion;” Huyghens's ”Traite de la Lumiere” was published in 1690. From that time the undulatory theory lost ground, until it was revived by Young and Fresnel. It soon after received great support from the establishment, by Joule and others, of the mechanical theory of heat. One remark of Young's respecting the ether opens up a question which has attracted much attention of late years. In a letter addressed to the Secretary of the Royal Society, and read January 16, 1800, he says:--

That a medium, resembling in many properties that which has been denominated ether, does really exist, is undeniably proved by the phenomena of electricity; and the arguments against the existence of such an ether throughout the universe have been pretty sufficiently answered by Euler. The rapid transmission of the electrical shock shows that the electric medium is possessed of an elasticity as great as is necessary to be supposed for the propagation of light. Whether the electric ether is to be considered as the same with the luminous ether--if such a fluid exists--may perhaps at some future time be discovered by experiment.

Besides his contributions to optics, Young made distinct advances in connection with elasticity, and with surface-tension, or ”capillarity.” It is said that Leonardo da Vinci was the first to notice the ascent of liquids in fine tubes by so-called capillary attraction. This, however, is only one of a series of phenomena now very generally recognized, and all of which are referable to the same action. The hanging of a drop from the neck of a phial; the pressure of air required to inflate a soap-bubble; the flotation of a greasy needle on the surface of water; the manner in which some insects rest on water, by depressing the surface, without wetting their legs; the possibility of filling a tumbler with water until the surface stands above the edge of the gla.s.s; the nearly spherical form of rain-drops and of small drops of mercury, even when they are resting on a table,--are all examples of the effect of surface-tension. These phenomena have recently been studied very carefully by Quincke and Plateau, and they have been explained in accordance with the principle of energy by Gauss. Hawksbee, however, was the first to notice that the rise of a liquid in a fine tube did not depend on the thickness of the walls of the tube, and he therefore inferred that, if the phenomena were due to the attraction of the gla.s.s for the liquid, it could only be the superficial layers which produced any effect. This was in 1709. Segner, in 1751, introduced the notion of a surface-tension; and, according to his view, the surface of a liquid must be considered as similar to a thin layer of stretched indiarubber, except that the tension is always the same at the surface bounding the same media. This idea of surface-tension was taken up by Young, who showed that it afforded explanation of all the known phenomena of ”capillarity,” when combined with the fact, which he was himself the first to observe, that the angle of contact of the same liquid-surface with the same solid is constant. This angle he called the ”appropriate angle.” But Young went further, and attempted to explain the existence of surface-tension itself by supposing that the particles of a liquid not only exert an attractive force on one another, which is constant, but also a repulsive force which increases very rapidly when the distance between them is made very small. His views on this subject were embodied in a paper on the cohesion of liquids, read before the Royal Society in 1804. He afterwards wrote an article on the same subject for the supplement of the ”Encyclopaedia Britannica.”

The changes which solids undergo under the action of external force, and their tendency to recover their natural forms, were studied by Hooke and Gravesande. The experimental fact that, for small changes of form, the extension of a rod or string is proportional to the tension to which it is exposed, is known as Hooke's law. The compression and extension of the fibres of a bent beam were noticed by James Bernoulli, in 1630, by Duhamel and others. The bending of beams was also studied by Coulomb and Robison, but Young appears to have been almost the first to apply the theory of elasticity to the statics of structures. In a letter to the Secretary of the Admiralty, written in 1811, in reply to an invitation to report on Mr. Steppings's improvements in naval architecture, Young claimed that he was the only person who had published ”any attempts to improve the _theory_ of carpentry.” It may be here mentioned that Young accepted the invitation of the Admiralty, and sent in a very exhaustive report, which their Lords.h.i.+ps regarded as ”too learned” to be of great practical value. Young's contributions to this subject will be chiefly remembered in connection with his ”modulus of elasticity.” This he originally defined as follows:--

”The modulus of the elasticity of any substance is a column of the same substance capable of producing a pressure on its base which is to the weight causing a certain degree of compression as the length of the substance is to the diminution of its length.”

It is not usual now to express Young's modulus of elasticity in terms of a length of the substance considered. As now usually defined, Young's modulus of elasticity is the force which would stretch a rod or string to double its natural length if Hooke's law were true for so great an extension.

So much of Dr. Young's scientific work has been mentioned here because it was during his early years of professional practice that his most original scientific work was accomplished. As already stated, after two years' tenure of the Natural Philosophy chair at the Royal Inst.i.tution, Young resigned it because his friends were of opinion that its tenure militated against his prospects as a physician. In the summer of 1802 he escorted the great-nephews of the Duke of Richmond to Rouen, and took the opportunity of visiting Paris. In March, 1803, he took his degree of M.B. at Cambridge, and on June 14, 1804, he married Eliza, second daughter of J. P. Maxwell, Esq., whose country seat was near Farnborough. For sixteen years after his marriage, Young resided at Worthing during the summer, where he made a very respectable practice, returning to London in October or November. In January, 1811, he was elected one of the physicians of St. George's Hospital, which appointment he retained for the rest of his life. In this capacity his practice was considerably in advance of the times, for he regarded medicine as a science rather than an empirical art, and his careful methods of induction demanded an amount of attention which medical students, who preferred the more rough-and-ready methods then in vogue, were slow to give. The apothecary of the hospital stated that more of Dr. Young's patients went away cured than of those who were subjected to the more fas.h.i.+onable treatment; but his private practice, notwithstanding the sacrifices he had made, never became very valuable.

In 1816 Young was appointed Secretary to a Commission for determining the length of the second's pendulum. The reports of this Commission were drawn up by him, though the experimental work was carried out by Captain Kater. The result of the work was embodied in an Act of Parliament, introduced by Sir George Clerk, in 1824, which provided that if the standard yard should be lost it should ”be restored to the same length,” by making it bear to the length of the second's pendulum at sea-level in London, the ratio of 36 to 391393; but before the standards were destroyed, in 1835, so many sources of possible error were discovered in the reduction of pendulum observations, that the Commission appointed to restore the standards recommended that a material standard yard should be constructed, together with a number of copies, so that, in the event of the standard being again destroyed, it might be restored by comparison with its copies. In 1818 Young was appointed Superintendent of the Nautical Almanac and Secretary of the Board of Longitude. When this Board was dissolved in 1828, its functions were a.s.sumed by the Admiralty, and Young, Faraday, and Colonel Sabine were appointed a Scientific Committee of Reference to advise the Admiralty in all matters in which their a.s.sistance might be required. The income from these Government appointments rendered Young more independent of his practice, and he became less careful to publish his scientific papers anonymously. In 1820 he left Worthing and gave up his practice there. The following year, in company with Mrs. Young, he took a tour through France, Switzerland, and Italy, and at Paris attended a meeting of the Inst.i.tute, where he met Arago, who had called on him in Worthing, in 1816. At the same time he made the acquaintance of Laplace, Cuvier, Humboldt, and others. In 1824 he visited Spa, and took a tour through Holland. In the same year Young was appointed Inspector of Calculations and Medical Referee to the Palladium Insurance Company. This caused him to turn his attention to the subject of life a.s.surance and bills of mortality. In 1825, as Foreign Secretary of the Royal Society, he had the satisfaction of forwarding to Fresnel the Rumford Medal in acknowledgment of his researches on polarized light. Fresnel died, in his fortieth year, a few days after receiving the medal.

Dr. Young died on May 10, 1829, in the fifty-sixth year of his age, his excessive mental exertions in early life having apparently led to a premature old age. He was buried in the parish church of Farnborough, and a medallion by Sir Francis Chantrey was erected to his memory in Westminster Abbey.