SECTION IV. DIFFRACTION. The modification called diffraction has now become one of the essential parts of Optics. It was entered upon by Grimaldi and Newton, advanced by the researches of Dr. Young, and completed by those of Fresnel. It consists of the deviation, always accompanied by a more or less marked dispersion, that light undergoes, in passing close by the edges of any body or opening. Its simplest way of manifesting itself is by the unequal and variously-coloured fringes, some exterior and some interior, which surround the shadows produced in a darkened room. The famous general principle of interferences, discovered by Dr. Young, is the most important idea connected with this theory. It was not appreciated, remarkable as it is, till Fresnel made use of it to explain several interesting phenomena, difficult to analyse; and, among others, the celebrated phenomenon of the coloured rings, which were by no means fully accounted for by Newton's admirable efforts. The law of interferences is this: that when two luminous cones emanate from the same point, and follow, for any reason, two distinct courses, but little inclined towards each other, the intensities proper to the two lights neutralize and augment each other alternately, increasing by equal and minute degrees, the value of which is determined, the difference in length between the entire paths traversed by the two cones. It is a pity that this important principle should have suffered, like the rest, from being implicated with chimerical conceptions on the nature of light. We have done all that the nature of this Work admits, in regard to Optics; and we must pass over the subjects of the double refraction proper to various crystals, the general law of which was discovered by Huyghens. We must also omit the phenomena of polarization, disclosed by Malus. In what I have brought forward, I hope that, while I have pointed out the gaps in this science, of which we are too little conscious at present, I have also placed in a clear light the great and numerous results obtained during the last two centuries, notwithstanding the disastrous preponderance of vain hypotheses about the nature of light over the spirit of rational experimentation. 300 History. CHAPTER VI. ELECTROLOGY. HIS last branch of Physics, relating as it THIS manifest phenomena, could not be developed till after the rest. The electrical machine indeed is as old as the airpump; but it was not till a century later that the study assumed a scientific character, through the distinction of the two electricities, Muschenbroek's experiments with the Leyden jar, and then through Franklin's great meteorological discovery, which was the first manifestation of the influence of electricity in the general system of nature. Up to that time, the isolated observations of philosophers had only suggested the character of generality inherent in this part of Physics, as in all others, by continually adding to the number of substances susceptible of electrical phenomena: and it was not till the end of the last century that this department of Physics presented anything like the rational character which belongs to the others. It is owing to the labours of Coulomb that it takes its place, and still an inferior place, with the rest. Condition. No other science offers so great a variety of curious and important phenomena; but facts do not constitute science, though they are its foundation and material. Science consists in the systematizing of facts under established general laws: and, regarded in this way, Electrology is the least advanced of all the branches of Physics, imperfect as they all are. In the absence of ascertained laws, arbitrary hypothesis has run riot. The simple confidence with which students have explained all phenomena by endowing imaginary fluids with new properties for every fresh occurrence, reminds us of the old metaphysical explanations, the ancient entities being Arbitrary hypotheses. HYPOTHESES ABOUT ELECTRICITY. 301 merely replaced by supposed fluids. But the delusion is less mischievous here than in Optics, where the arbitrary conjectures are closely connected with real laws, and share their imposing character. In electrology the hypotheses, standing alone, exhibit their barrenness; and everybody can see that they have borne no share in the great discoveries of the last half-century, though the discoveries, once made, have been afterwards attached to the hypotheses. Most people regard them now as a sort of mnemonic apparatus, useful for connecting facts in the memory, though originally designed for a very different purpose. They are a bad apparatus for even this object, which would be much better answered by a system of scientific formulas especially adapted to that use. And, though less mischievous than in Optics, hypotheses of this order do harm in electrology, as everywhere else, by concealing from most minds the real needs of the science. It should be remembered, moreover, that anti-scientific action like this extends its influence over the succeeding and more complex sciences, which, on account of their greater difficulty, require the severest method, the type of which will naturally be looked for in the antecedent sciences. It is a serious injury to transmit to them a radically vicious model. While physicists are using these hypotheses as having avowedly no intrinsic reality, their very use leads students of the successive sciences, and especially physiologists, to consider them the very sublimity of physics, and to proceed to take them for the bases of their own labours. We see how the notion of magnetic and electric fluids tends to confirm that of a nervous fluid, and to encourage wild dreams about the nature of what is called animal magnetism, in which even eminent physicists have shared. Such consequences show how a study which is naturally favourable to the positive development of human intelligence may, by vicious methods of philosophizing, become fatal to our understandings. From the complex nature of the phenomena, there can be but little application of mathematics in electrology. It has as yet Relation to borne only a small share in the progress of the science: but it is as well to point out the two ways, the one illusory, the other real,-in which the application of mathematics has been attempted. Unsound application. Sound application. Those who have occupied themselves with imaginary fluids as the causes of electrical and magnetic phenomena, have transferred the general laws of rational mechanics to the mutual action of their molecules; thus making the body under notice a mere substratum, necessary for the manifestation of the phenomenon, but unconcerned in its production; with which office the fluid is charged. It is clear that mathematical labours so baseless can serve no other purpose than that of analytical exercise, without adding a particle to our knowledge. In the other case, -of a sound application,—the mathematical process has been based on some general and elementary laws, established by experiment, according to which the study of phenomena proper to the bodies themselves has been pursued,-all chimerical hypotheses being discarded. This is the character of the able researches of M. Ampère and his successors, on the mathematical investigation of electro-magnetic phenomena, in which the laws of abstract dynamics have been efficaciously applied to certain cases of mutual action between electric conductors or magnets. In examining the principal parts of electrology, we must exclude all that belongs to the chemical or physiological influence of electricity, and all connection of electricity with concrete physics; and especially with meteorology. Thus limited to the physical and abstract, Divisions. electrology at present comprehends three orders of researches. The first relates to the production, manifestation, and measurement of electrical phenomena: the second, to the comparison of the electric state proper to the different parts of the same mass, or to different contiguous bodies: the third, to the laws of the motions which result from electrization: we may add, as a fourth head, the application of the results under the other three to the special study of magnetic phenomena, which can never henceforth be separated from them. |