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REV. THOMAS WILLIAM WEBB, M.A., F.R.A.S.
PREBENDARY OF HEREFORD CATHEDRAL;
AUTHOR OF “CELESTIAL OBJECTS FOR COMMON TELESCOPES.
PUBLISHED UNDER THE DIRECTION OF THE COMMITTEE OF GENERAL
FOR PROMOTING CHRISTIAN KNOWLEDGE,
43, QUEEN VICTORIA STREET, E.C.;
BRIGHTON: 135, NORTH STREET.
I PROPOSE to give you a little information in the science of Optics. Optics means the knowledge of the nature and properties of Light. It is a very difficult subject in its full extent, and requires a deep knowledge of mathematics. But much that is not only very useful but very interesting is within the reach of any one who will take a little pains; and I think I can promise him that he will not be disappointed. There is never any disappointment in the reverent study of the great works of GOD. And what was His first recorded work? He said, Let there be Light! And well might He thus open His wonderful and beautiful creation. We are so accustomed to the blessing of Light that we are scarcely able to value it as we ought. But let us try to think what our condition would be were light to be withdrawn from us-were the sun to be darkened in the midst of heaven, and every artificial source of light to fail. It is a question whether life itself could in such a case be long maintained, so intimate is the connection of light with all the processes of animal and vegetable existence; but at any rate it could only be a condition of comparative misery. Or, to take another illustration; we pity the blind, and well we may: and it is our duty to do everything to alleviate so sad a privation. But what if we were all blind together, even from our birth? Surely we do not speak amiss in saying that Light is one of the very best and chiefest gifts of Him who is pleased to describe Himself under the name of Light. To light we owe by far the greater part of all that is valuable and all that is lovely in this most beautiful world.
But what is Light? We may say that it is that by which we see. But what is its nature? The true answer is probably far beyond the capacity of such a poor ignorant creature as man. We know nothing of the true nature of gravity, or electricity, or chemical action, any more than of light. All we can do is to seek for such an explanation as will account best for the properties of light, and to content ourselves with that till we are permitted to find a better. Such an explanation is given in a very complete and remarkable manner by what is called the Undulatory Theory of Light. This supposes light to consist of undulations or waves 1, so minute and so rapid as to pass our power of conception. Still, we had better try. We may get some notion in this way. We can easily divide an inch into 10 parts. The common carpenter's rule will not help us, because he wants half-quarters, or eighths. But we shall find the inch divided into tenths on the scales in the cases of mathematical instruments. Now let us take a straight narrow strip of paper or card, and by means of the scale, mark off upon it the tenth of an inch. This we can easily cut in half crosswise with a fine pair of scissors; and with sharp eyes and steady fingers we may manage to cut one of these halves across again into 5 parts; each of these hair-breadth strips will then be about both of an inch wide. Now, who could cut one of these into 10 again? If it were done, we should scarcely see them without a microscope. And
1 The undulatory theory, previously suggested by Hooke, the able contemporary of Newton, more clearly developed by Huygens, Fresnel, and others, and established by the researches of Dr. Young, has gradually supplanted that advocated by Newton. The supposition of waves or vibrations implies of course the further supposition of a medium in which they exist. This medium is called the ether or luminiferous ether; but its existence being only assumed for the sake of the theory, it need not be further noticed.