about the height of the jar by 6 inches or 7 inches wide, are arranged perpendicularly behind the tube, inclosing it as it were within a right angle, though not touching, they give, of course, by reflection, two additional images of the illuminated tube; but each of these, since the light leaves the tube from a different side, exhibits when the quartz is used a different colour, all three changing to successive colours with the rotation of the Nicol. With a large Nicol polariser embracing the full parallel beam from the lantern, the effect is finer still, and may be varied by employing a jar of rather greater diameter, and throwing the polarised light down through two apertures covered with quartzes of opposite rotations. In this case, in all but two positions of the analyser, there will be two beams of light in each image of the jar, glowing with different colours.

191. Identity of Light, Heat, and Actinism.That the heat rays and chemical rays are subject to the same laws as luminous rays, as regards reflection, refraction, and dispersion, has been already stated (§ 88), and is a familiar truth proved in every camera every day. It only remains to state that they obey also the laws of polarisation and double refraction. If the two images which have passed through a double-image prism are tested with a thermopile, this is readily demonstrated, as is the fact that the ray is quenched whenever polariser and analyser are crossed. The actinic rays may be similarly tested with a sensitive plate, thus making the demonstration complete as regards all the rays of the visible and invisible spectrum, and proving that the sole difference between any of them is in period of vibration; some periods being more active in certain ways and some in others. Captain Abney has very recently shown that it is possible to obtain in a dark room a photographic image of a kettle heated far short of the luminous degree; or on the other hand, to impress a

sensitive plate with a photographic image of large portions of the spectrum through an apparently opaque plate of ebonite. And Professor Tyndall has carried the demonstration to the last degree of refinement, by proving experimentally that a plane-polarised beam of dark heat, filtered of all visible rays by a solution of iodine in bisulphide of carbon, is rotated, like the luminous rays (§ 150), by a powerful electric current, or when the glass or other diathermous material is placed in a magnetic field.

CHAPTER XVIII.

CONCLUSION.

THROUGH all the experiments now described, we have discovered that the phenomena and sensations we know as Light and Colour, when traced back and examined, found their ultimate explanation in forms of Motion. We were shut up very early to that conclusion: we were absolutely compelled to travel in our thoughts from what we saw" to something we could not see at all, and to form mental images of invisible waves, whose undulations were propagated with incredible swiftness all around us. Later on we found phenomena which appeared to reveal to us the actual and precise directions, or orbits, of the vibrations in those waves; and applying to that hypothesis delicate and beautiful experimental tests, such as can be readily understood by any educated mechanic or other intelligent reader, we found our supposed orbits respond to those tests in every particular. The motions were, so far as we could judge from any possible mode of examination, modified, varied, resolved, or compounded, in all respects as our hypothesis led us to expect. This is the nature of the evidence, and we have thus reviewed in actual experiment the principal facts, on which is built up the Undulatory or Wave Theory of Light. The profoundest mathematical researches, applied to the most refined

experiments varied in every possible way, have so far only confirmed that theory in every particular.

Let us fully grasp the grand conception; for there is no grander throughout the entire material Universe! All around us-everywhere-space is traversed in all directions by myriads of waves. Not more surely does a nail take up from a hammer the force of a blow, than does each particle of Something take up and pass on the motion of the preceding particle. Heat, Light, Colour, Electricity-all alike are simply propagations of disturbance through that Something which we call Ether. Invisible themselves, these wonderful motions make all Things visible to us, and reveal to us such things as are. Take away from the diapason of these invisible waves those of any given period, and if we lose the dazzling whiteness which results from them all in due proportion, we but increase the soft splendour of the phenomena, as the hues of the rainbow appear before our eyes. Let them clash against, oppose, and so destroy one another; and even their very interferences, though dark shadows may cross our vision, produce amidst these forms and colours of almost unearthly beauty. Motion in the Ether accounts for all.

But we have taken another step from the seen to the unseen; for we have conceived and named this Ether. The name is of course nothing: but we cannot do without the thing itself-we must conceive it. No eye has seen it; no instruments can weigh it; no vessel can contain it; nothing can measure it; yet it must be there. "There?"-yea, here also, and everywhere. Absolutely invisible, it yet is the sole key to all physical phenomena; and the most recent, most widely received, and altogether most probable theory about Matter itself, is that its atoms are but Vortices in its infinite bosom. Ask for "absolute proof" of its verity, and there is absolutely none; and there are even about the conception itself some stupendous difficulties. The physicist has to

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endow his Ether with the most contradictory properties: he conceives it as rarer and more subtle than the most exhausted atmosphere, with the principal properties of a perfectly elastic fluid, and yet, withal, the chief distinguishing property of a solid! All these things do not deter him; and he believes implicitly in this Ether he has never seen and never will see, simply because without it he can explain no solitary phenomenon around him, while with it and its motions he can explain everything. Light is thus to him a Revealer of all Nature, both visible and invisible.

Another step further yet. The inquiry is irresistibly suggested, whether the comparison and the analogy may not go further, and afford us some revelation deeper still. That inquiry is strictly legitimate. If our universe be in truth an objective and conditioned manifestation of any absolute Source of all being, it should be thus; the Actual ought, in its limited measure, to reveal to us truly the Essential and Eternal. The student of nature, at all events, does hold expressly that if Nature has any Author, she must speak aright of Him if she speak at all; and as for the so-called religious man, while any book can only take a secondary place in such an inquiry as this, he also believes that it ought to be thus, since his book actually says so. The point of surpassing interest therefore is, whether as regards this question there is any definite agreement between these two, as to which Science can have anything to say, or possesses any means of judging.

What then do we find? We are bound here at least to ask the expounders of physical science first, for every reason. We inquire, therefore, what purely physical science, and experiment, and speculation-what they at present appear to teach us?

1. They tell us of an intangible, invisible Ether, which cannot be touched, or tasted, or contained, or measured, or