THE QUARTERLY JOURNAL OF SCIENCE. JANUARY, 1869. I. THE ETHEREAL HYPOTHESIS OF LIGHT. By JAMES SAMUELSON, Editor. THE thirst for knowledge in the human mind is as insatiable as the wants of an immortal soul are necessarily unlimited. There are indeed myriads, content to go their daily rounds and confine their inquiries to the price of corn, cotton, consols, or whatever staple may serve to provide them with the necessaries and luxuries of life; but there are nobler men than those, who would rather be the discoverers of a secret in nature that yields wealth to thousands, than one of the ignorant thousands who reap the fruits of their researches; and of such men none have shown themselves more disinterestedly devoted to their intellectual calling than the students of physical and chemical science. Indeed it is almost to be regretted that they are not a little more worldly, for in that case their scientific theories and speculations would probably rest upon a more material basis than they sometimes do at present. Amongst the numerous subjects which are now engaging the attention of physical philosophers, there is none, perhaps, of deeper interest either to scientific men or to the lovers of the mysterious in nature, than that which relates to the illimitable space, wherein the universe of suns and planets moves incessantly, which serves as the medium to convey intelligence from sphere to sphere, and to communicate life from the great centres to the surrounding orbs. But it is rather as a curious inquirer, than with any pretensions to original research; rather in the hope that my observations and criticisms may stimulate discussion and cause further investigation, than with any expectation that they will throw fresh light upon so difficult and obscure a cosmical inquiry, that I propose its consideration in the present article. There are in the present day two distinct theories in relation to light, arising not from any difference of opinion as to the action of VOL. VI. B that force, inasmuch as the undulatory theory is now pretty generally accepted, but from opposing views as to the medium upon and through which it operates. It is needless for me to dwell long upon the undulatory theory, but, for the government of those who have not devoted much attention to the subject, it may be as well to mention that formerly light was not regarded as a force acting upon matter, but was supposed to consist of particles or atoms emitted by the luminous body, whilst electricity was considered an imponderable fluid which travelled through the substances electrified. Now, strangely enough, the views of the more advanced physical philosophers are to some extent reversed. Mr. Grove considers the electrical spark, at least, to consist generally of projected particles of the electrified substance,* and he gives apparently satisfactory reasons for so doing, whilst Professor Tyndall thinks that electricity may be a force acting upon "condensed ether which surrounds the atoms" of matter.† And on the other hand, as we shall presently find, the latter entirely discards the notion of any known substance as the vehicle of light, whilst Grove considers it to be a force acting upon gross but highly attenuated matter. But as I have already said, all are agreed upon the dynamical theory of light, first propounded by Huyghens in Newton's time, and afterwards supported and established in this country by Dr. Young; and this theory attributes to light a similar, though not exactly the same property, as sound, regarding it as a force which causes undulations of marvellous rapidity in the medium through which it travels. In the case of sound, the passage of the force is admitted on all hands to be through known matter, and it is well known that a vacuum is incapable of transmitting sound; but in that of light, which passes from sphere to sphere in the universe, and traverses a vacuum with apparently greater facility than air, it is obviously necessary either to discover or to suppose a medium for its transmission. That there is such a medium in interplanetary space is most probable, for light occupies time in its passage, corresponding with the distances between the luminous bodies from which it emanates and the spheres it illuminates, and therefore (in the case of our sun and earth for example) it cannot be the atmosphere alone which offers resistance to its passage. There is most likely matter of some kind, however attenuated, in space; and this is shown, not alone by the impeded passage of light, but by the retarded motions of the comets. * But what is that interplanetary matter? "The electric spark, the brush, and similar phenomena, the old theories regarded as actual emanations of the matter or fluid Electricity, I venture to regard them as produced by an emission of the material itself from whence they issue, and a molecular action of the gas, or intermedium, through or across which they are transmitted."-Correlation of Physical Forces and Continuity,' p. 112. 5th edition. Longmans: see also p. 181. 4 Heat as a Mode of Motion,' p. 216, note. 2nd edition. Longmans. Is it, as Professor Tyndall supposes, a specific "ether," which serves as the vehicle for light and electricity? Does it enter, as he believes it does, into the constitution of material bodies? or is it excluded beyond the limits of our atmosphere? Or again, is it, as Mr. Grove believes, an attenuated gas or mixture of gases, given off from the atmospheres of the revolving worlds? Let us first endeavour to understand the conceptions of these opposite thinkers, and then to test their respective hypotheses by the best means at our command. Professor Tyndall's conception of a "luminiferous ether" is that it is "a substance almost infinitely elastic," filling all space as with "jelly."* It fills up the interstices between molecules of all kinds of matter, "suffering no rupture of continuity at the surface of the eye, the intermolecular spaces of the various humours being filled with it." He believes it to form the infinite ocean in which worlds move, and to be the medium for the transmission of light there, as well as in the intermolecular spaces of material substances-in short he regards it as the medium for the transmission of light-(and probably of electricity) everywhere. Mr. Grove objects to this idea of a specific ether, both for the transmission of light and electricity. His views concerning the latter force we have given generally,§ and his ground for refusing to accept the doctrine in regard to light is, that "the more porous bodies, or those most permeable by ether, should be the best conductors," and that "an objection immediately occurs in the opacity of porous, and transparency of certain dense bodies.¶ He believes in the universality of ordinary matter, however attenuated, and considers his hypothesis "the least gratuitous.”** There are other writers, who, seeking to reconcile these opposite views, suppose that the ether does not penetrate our atmosphere, being "non-miscible" with it, and that therefore it does not permeate terrestrial matter.tt This hypothesis may be at once dismissed, for if the supposed ether is not miscible with our atmosphere, then the latter should itself be the medium upon which light operates; therefore the first stroke of the piston of an air-pump should cause the receiver to darken, and an object in an exhausted receiver should be invisible, just as the sound of a bell striking therein is inaudible. In the present state of the discussion and of our knowledge, therefore, we are left to consider the respective merits of the two hypotheses, Heat as a Mode of Motion,' p. 254. +On Radiation,' p. 9. Longmans. Correlation and Continuity, p. 133-4. They will be found detailed in the chapter on "Electricity" in his work on the Correlation of the Physical Forces.' ** Ibid., p. 186. Correlation,' p. 148. Ibid., p. 168. ++ Brooke's edition of Golding Bird's Natural Philosophy.' Sixth edition, p. 576. Churchill. which for convenience I shall call those of Grove and Tyndall; and in order to guide the students of various branches of physical science in their investigations, I propose, first, to select a few phenomena for the consideration of the micro-zoologist, chemical and physical experimenter, and mineralogist, and then to point out what appears, in my humble judgment, to be the inference deducible from those phenomena, leaving it to each class of observers to consider the value and accuracy of my investigations, and of the conclusion to which I have been led by them.† First, then, it is familiar to all who have any knowledge of natural history that the brilliant hues of the Lepidoptera, or Butterflies, are due to innumerable minute scales, regularly disposed upon their wings, just as the feathers of birds impart the bright colours to those races. In certain butterflies the wings have an iridescent, or metallic lustre (Lycaena Adonis, the Clifton Blue); in others it is dead and velvety (Vanessa Io, the Common Peacock); whilst in others again, both appearances are intermingled (Polyommatos Phloas, the small copper). Now let us inquire to what cause this phenomenon is attributable. We will take a specimen of Lycona Adonis, the Clifton Blue, of which the blue is quite metallic, or satiny, if I may be allowed to coin the word, and on placing a few of the scales of this insect under the microscope and examining them by transmitted sunlight, that is, by light reflected from the mirror and transmitted through the scales to the eye, we shall find certain of them quite crystalline and transparent (see Plate, Fig. 1); others bright orange-red (Fig. 2); and others again dusky brown, almost approaching to black. Now let us close off the light reflected from the mirror and examine the same scales by incident light, that is, by light concentrated upon them with the aid of the bull's-eye lens, and we shall find those which by transmitted light appeared translucent and colourless (Fig. 1) to be greenish brown or grey, studded with bright spots (Fig. 1a); those which were orange-red by transmitted light (Fig. 2) now appear of a brilliant violet-blue (Fig. 2a), the characteristic blue of the wings themselves; whilst the dull brown * I call the "ethereal" theory Professor Tyndall's, inasmuch as he has sought to develop it; but those who are interested in its origin and history may refer to that author's work, 'Heat as a Mode of Motion.' Professor Faraday appears to have given a cautious, or perhaps I should say partial, adhesion to the theory; and he refers to it once or twice in passing, in the Bakerian Lecture of 1851, which I shall quote freely in this article. See also Tyndall's 'Faraday as a Discoverer,' p. 129. Longmans. † Mr. Clerk Maxwell, it may be mentioned, considers "light" as a mode of electro-magnetic motion. He says, it "consists of alternate and opposite rapidly recurring transverse magnetic disturbances, accompanied with electric displacements, the direction of the electric displacement being at right angles to the magnetic disturbance, and both at right angles to the direction of the ray." (Proceedings of the Royal Society, 1864.) |