Page:The American Cyclopædia (1879) Volume X.djvu/442

 436 LIGHT can be given than by quoting a few words from Huygens's Traite de la lumilre (1690; Tracta- tus de Lumine, 1728). It will also thus be seen that he had formed views in regard to mo- lecular physics which have but recently been adopted, and that he had a pretty clear idea of the doctrine of dissociation almost a century and a half before the birth of Sainte-Claire- Deville, its commonly reputed author. " No one will question," says Huygens, " but that light consists of a motion of a certain matter as regards its effects. It appears that light, when gathered into a focus by a concave mirror, has the property of burning like fire, that is to say, it dissociates the particles of bodies (quod disjungat partes corporum) ; and this most cer- tainly indicates motion, at least according to that philosophy wherein the causes of all nat- ural effects are conceived by means of mechan- ical reasons. ... If we consider with what great velocity the rays of light are propaga- ted on all sides, and how, setting out from vari- ous and even opposite quarters, they intersect without interfering with one another, we will easily understand that lucid bodies are not seen by means of a certain luminous matter coming from them to us, as a ball or an arrow passes through the air. ... It therefore moves in another way, and to understand this it will be well to know how sound passes through the air." He then gives the explanation of the propagation of sound in a manner scarcely equalled by any modern writer, and proceeds : "There is no doubt but that light reaches us from luminous bodies by means of motion given to the interposed matter. . . . Light and sound, though they possess successive motion in common, yet differ very widely in other re- spects, as the motion which is the cause of each is differently produced, and the matter is dif- ferent in which the motion takes place, and the mode different whereby the motion is communicated. For sound has for its cause a sudden concussion of the whole body, or of a large part of it, which puts the contiguous air in motion ; but light must arise from the sep- arate parts of the luminous body, so that they are all plainly seen. ... In luminous bodies the motion is produced by a violent concussion of the particles, whereby an impulse is given to the ethereal matter. If we now inquire what is that ethereal matter wherein that motion springs, it will be seen that it is not the same as that which serves for the propagation of sound; for this is no other than the air we breathe, which being removed, the other still remains, a fact proved by placing the sonorous body in a glass vessel and removing the air by Boyle's machine." It is sometimes said that Huygens entertained the idea that light was propagated in the luminiferous ether in the same manner that sound is in the air, that is, by to and fro vibrations in the line of propa- gation ; but according to the above quotation, and for other reasons, this conclusion is scarce- ly well founded. He does indeed compare the action to that which accompanies the impact of elastic bodies, but does not suggest any defi- nite method of production of the vibrations. He concerns himself more with the forms of the wave fronts which are produced by the vibrations, and in that way arrives at mathe- matical results which, by the most rigid ex- perimental and theoretical tests, have been found true. The composition of light by the union of rays of different degrees of refrangi- bility was not then known, and the cause of this difference of refrangibility not till more than a century after; several other of the phenomena of light, as interference and dif- fraction, had not been well observed, and re- quired some additional hypotheses for their explanation ; but the fundamental principles which enter into the explanation of all the phe- nomena, then as well as recently observed, were laid down by him, and will never be affected by any changes of hypothesis in regard to the precise mode of motion of the individual par- ticles of ether. The additional hypotheses by which the theory has been brought to more completeness, the most important of which is that of transverse vibrations and the deduced principle of interference, were proposed (1801 '3) by Dr. Thomas Young of England, and hence many of his countrymen regard him as really the founder of the undulatory theory. The principle of interference, however, was not perfectly established and generally applied until Fresnel brought to bear upon the sub- ject the analytical powers of his great mathe- matical genius. Euler, Mains, Cauchy, Arago, Biot, Sir David Brewster, Sir William Hamil- ton, Sir Gr. B. Airy, and other investigators have also added many important contributions. The definite conception of transverse vibra- tions of different lengths by which the rays of different refrangibility were propagated must, however, be considered as an important part of the undulatory theory as it now stands, and the principal hypothesis upon which a great share of the physical explanations depends. It may seem remarkable that this theory, whose fundamental laws were so clearly stated nearly two centuries ago, should not have been sooner accepted, as it is thought the emission theory can be put to the test of direct experiment. Experiments made by Mr. Bennett are pointed to as being conclusive. He suspended a slen- der straw horizontally by means of a spider's web, and attached a piece of white paper to one end of the delicate balance. He then in- troduced it into the receiver of an air pump, exhausted the air, and brought the focus of a powerful lens to Bear upon it, but without producing any motion in the ponderable mat- ter of the balance. It is asserted that on the emission theory the immense velocity of the luminous particles, although they might be infinitesimally small, would possess sufficient momentum to impart a sensible degree of mo- tion to light bodies; and the position cannot well be denied. If a single molecule of light