Dr. Thomas Young, who had supported with great ingenuity and force of argument the undulatory theory of light, as maintained by Hooke and Huygens, was the first who gave a plausible explanation of the inflection of light. By interposing a small screen at B, [fig. 10], and intercepting the rays that passed near the hair X, he was surprised to find that all the fringes within the shadow disappeared. The same effect took place when the screen intercepted the rays on the other side; and hence he concluded, that the rays on each side of the hair were necessary to the production of the inner fringes, and that the fringes were produced by the interference of the rays that passed on one side of the hair with those that passed on the other side. In order to account for the coloured fringes without the shadow, Dr. Young conceived that the rays which pass near the edge of the hair interfere with others, which he supposes may be reflected after falling very obliquely upon its edge,—a supposition which, if correct, would certainly produce fringes very similar to those actually observed.
In pursuing these researches so successfully begun by Dr. Young, M. Fresnel had the good fortune to explain all the phenomena of inflection by means of the undulatory doctrine combined with the principle of interference. In place of transmitting the light through a small aperture, he caused it to diverge from the focus of a deep convex lens, and instead of receiving the shadow and its fringes upon a smooth white surface, as was done by Newton, he viewed them directly with his eye through a lens placed behind the shadow; and by means of a microscope he was able to measure the dimensions of the fringes with the greatest exactness. By this mode of observation he made the remarkable discovery, that the inflection of the light depended on the distance of the inflecting body from the aperture or from the focus of divergence;[31] the fringes being observed to dilate as the body approached that focus, and to contract as it receded from it, their relative distances from each other, and from the margin of the shadow continuing invariable. In attempting to account for the formation of the exterior fringes, M. Fresnel found it necessary to reject the supposition of Dr. Young, that they were owing to light reflected from the edge of the body. He not only ascertained that the real place of the fringe was the 17/100th of a millimetre different from what it should be on that supposition, but he found that the fringes preserved the same intensity of light, whether the inflecting body had a round or a sharp edge, and even when the edge was such as not to afford sufficient light for their production. From this difficulty the undulatory theory speedily released him, and he was led by its indications to consider the exterior fringes, as produced by an infinite number of elementary waves of light emanating from a primitive wave when partly interrupted by an opaque body.
The various phenomena of inflection, which had so long resisted every effort to generalize them, having thus received so beautiful and satisfactory an explanation from the undulatory doctrine, they must of course be regarded as affording to that doctrine the most powerful support, while the Newtonian hypothesis of the materiality of light is proportionally thrown into the shade. It is impossible, indeed, even for national partiality to consider the views of Newton as furnishing any explanation of the facts discovered by Fresnel; and, as no attempt has been made by the small though able phalanx of his disciples to stay the decision with which, on this count at least, the doctrine of emission has been threatened, we shall venture to suggest some principles by which the refractory phenomena may perhaps be yet brought within the pale of the Newtonian theory.
That the particles of light, like those of heat, are endowed with a repulsive force which prevents them from accumulating when in a state of condensation, or when they are detained by the absorptive action of opaque bodies, will be readily admitted. By this power a beam of light radiating from a luminous point has, in every azimuth, the same degree of intensity at the same distance from its centre of divergence; but if we intercept a portion of such a beam by an opaque body, the repulsive force of the light which formerly occupied its shadow is withdrawn, and consequently the rays which pass near the body will be repelled into the shadow, and will form, by their interference with those similarly repelled on the other side, the interior fringes, which are parallel to the edges of the body. The rays which pass at a greater distance will in like manner be bent towards the body, but with less force, and, interfering with those rays which retain their primitive direction, from the state of their fits or the position of their poles, they will form the exterior fringes. When the inflecting body is placed near the point of divergence, the greater proximity of the rays will produce a greater repulsive force, and consequently a greater inflection of the passing light; while the removal of the body from the point of divergence will be accompanied with an increased distance of the particles, an inferior repulsive force, and a feebler inflection. As the phenomena of inflection, considered under this aspect, arise from a property of the light itself, it follows that they will remain invariable, whatever be the nature or density of the body, or the form of the edge which acts upon the passing rays.
CHAPTER IX.
Miscellaneous Optical Researches of Newton—His Experiments on Refraction—His Conjecture respecting the Inflammability of the Diamond—His Law of Double Refraction—His Observations on the Polarization of Light—Newton’s Theory of Light—His “Optics.”
Before concluding our account of Newton’s optical discoveries, it is necessary to notice some of his minor researches, which, though of inferior importance in the science of light, have either exercised an influence over the progress of discovery, or been associated with the history of other branches of knowledge.
One of the most curious of these inquiries related to the connexion between the refractive powers and the chymical composition of bodies. Having measured the refractive powers and the densities of twenty-two substances, he found that the forces which reflect and refract light are very nearly proportional to the densities of the same bodies. In this law, however, he noticed a remarkable exception in the case of unctuous and sulphureous bodies, such as camphire, olive oil, linseed oil, spirit of turpentine, and diamond, which have their refractive powers two or three times greater in respect of their densities than the other substances in the table, while among themselves their refractive powers are proportional to their densities, without any considerable variation. Hence he concluded that diamond “is an unctuous substance coagulated,”—a sagacious prediction, which has been verified in the discoveries of modern chymistry. The connexion between a high degree of inflammability and a great refracting force has been still more strongly established by the high refractive power which I detected in phosphorus, and which was discovered in hydrogen by MM. Biot and Arago.
There is no part of the optical labours of Newton which is less satisfactory than that which relates to the double refraction of light. In 1690, Huygens, published his admirable treatise on light, in which he has given the law of double refraction in calcareous spar, as deduced from his theory of light, and as confirmed by direct experiment. Viewing it probably as a theoretical deduction, Newton seems to have regarded it as incorrect, and though he has given Huygens the credit of describing the phenomena more exactly than Bartholinus, yet, without assigning any reason, he rejected the law of the Dutch philosopher, and substituted another in its place. These observations of our author form the subject of the twenty-fifth and twenty-sixth queries at the end of his Optics, which was published fourteen years after the appearance of Huygens’s work. The law adopted by Newton is not accompanied with any of the experiments from which it was deduced; and though he has given it without expressing any doubt of its accuracy, it is, nevertheless, entirely incompatible with observation, and has been rejected by all succeeding philosophers.