UNDULATORY THE":',RY OF LIGHT. Optics ranks next to dynamics in the category of nearly eaoct knees—t hat is, of sciences whose lundamenta I principles are so well know n, that the result of almost any new experimental con bin at ion can be predicted metheinatieally. Given the lorees acting on a body, the laws of motion (q.v.)enable us, 1 y purely mathematical processes, to determine the consequent motion. Though we have not as yet arrived at equal perfection in optics, weave certainly far on the way, and r,b.ibly have now attained nearly all the progress (independent of improvements in oar 1: nth, on:tic:II methods) which will be made until the next great step in molecular physics :.1::111 give us the clue to the nature of the minute motions on wl.b 11 light, heat, electric cut rents, and magnetism depend. The most extraordinary and almost incredille pre Of ibeOry ba ye been verified by experiment. and at present the differences 1 etw•en the ry and experiment may he divided into two classes, corresponding to the : lowe exceptions. The first are those depending on the imperfections of mathematical here, because, for example. as we are yet unable to obtain the exact solution cermin utlfcrc utial equation, we have to content ourselves with an approximate one. I lit every impim einem in our means of approximation is found to introduce a closer (inept between theory and experiment. This difficulty may safely be left to mathe L :oh inns. It is otherwise with the second difficulty. This depends on our ignorance it the ultimate nature of matte], and cur consequent inability to apply mathematical ..rcrMg in a perfectly correct and sufficiently comprehensive manner. Here the work is still required, and it is in this directiout hat we must in all prob ability now k•ele for in•pert:int extensions of our knowledge.
Optics is divided into two parts, physical and geometrical. Of these, the latter con tents itself with rssuming certain obvious experimental truths, such as the fact, that light in a uniform medium moves in sn night lines, the ordinary laws of reflection and refract ion, etc., and, making these its basis, employs mathematics to develop their further consequences. It is thus that theory has shown how to carry to their utmost perfection inch exquisite specimens of art as the best telescopes and mic:oscopes of the present day. But these investiemtions, and their practical application, are wholly independent
of the nature of 1 ght, and cannot be affected by discoveries iu that direction.
It is otherwise when we come to physical optics. This commences with the ques tion: " 1I7 at 1. Light ?" and endeavors to deduce from the nature of light the experi mental laws ‘Vhieh. as We have seen, are assumed as the basis of geometrical optics.
By two perfectly distinct classes of astronomical observations—aberration (q.v), and the eclipses of Jupiter's satellites—we known that light takes time to pass from oue body to another—the velocity, however, being_ enormous—about 200,000 miles per second. Hence it follows. that either matter (q.v.) or energy (see Force) must be transferred from a Itchy to the eve before we can see it. Here we have at once the rival physical theories of light, which have alternatively had the advantage of one another in explaining observed phenomena. It is only of late years that an experimentum Cr 21CiS has finally duelled between them—by showing one of them to be utterly incompatible with a result of observation.
Newton adopted the corpuscular theory, in which light is supposed to consist of material particles-1 e.. lie adopted the first of the two possible hypotheses; and he gave the first instance of the solution of a problem involving molecular forces, by deducing from this theory the laws of reflection and single refraction. We shall see immediately that this beautiful investigation led to the destruction of the theory from which it was deduced. But, independent of this, there are many glare and obvious objections to the corpuscular theory; for it involves essentially the supposition of material particles impirering on the eye with the velocity of 200,000 m. per second! If such particles weighed but the millionth of a pound, encli would have something like ten times the momentum (q.v.), (i.e.. the battering power). and six million times the vis-viva (see Wortx). or kinetic energy (i.e., the penetrating, power), of a rifle-bullet. Suppose them a million times smaller yet as millions of millions of them must be supposed to enter the eye at once, coming from every point of the surface of every visible object, it seems impossible to reconcile such a hypothesis with the excessive delicacy of the organs of vision.