PREFACE.

VERY recently, at the Annual Meeting (1882) of the Teachers' Training and Education Society, Prof. Huxley reiterated, in his capacity of an Examiner in the Science and Art Department, a complaint well known at South Kensington as to the great difference between two kinds of "what persons called knowledge." He remarked for the hundredth time, how "That which they had constantly to contend against in the teaching of science in this country was, that many teachers had no conception of that distinction; for they thought it quite sufficient to be able to repeat a number of scientific propositions and to get their pupils to repeat them as accurately as they themselves did. . . . The teacher should be instructed that his business in teaching was to convey clear and vivid impressions of the body of facts upon which the conclusions drawn from those facts were based." To do this in some degree for one branch of Physics is the sole object of this unpretending little work; which hopes to be rather a handbook, companion,

and supplement to the excellent existing text-books, than anything else. With perhaps one exception in a short explanation of the law of sines, no set formula whatever will be found in it; but I have merely tried to place clearly before the mind of the reader, though something like a complete course of actual experiments, the physical realities which underlie the phenomena of Light and Colour. As helps, there are solely employed simple mechanical analogies and a few diagrams, explained in language which it is hoped may be found in reality simple and clear, though not intended to be childish, or to debar any private student from the healthful exercise of now and then considering what the writer means.

Now in carrying out a course of experiments in Physical Optics, projection upon a screen is not only far superior in general effect to any other method of demonstration, besides having the advantage of exhibiting the phenomena to the whole of a class or audience at the same time, but has another recommendation of primary importance. The trained physicist well understands the meaning of the rays which enter his eye, and are visible to him solely, direct from his lens, prism, or other apparatus; but the scholar new to the subject finds it very difficult to interpret in terms of physical phenomena what thus appeals merely to his own visual impressions. When he looks through a prism, for instance, it is difficult for him to get rid of the

he sees.

vague notion that "something in the prism" colours what But when actual rays of light are projected through the prism, and the colours appear on the screen apart from himself, as it were, he cannot help understanding that what thus appears to others at the same moment as to him is a physical reality which he has to trace out, and which he must reduce to physical terms. What is meant will be

readily understood by any science-teacher.

Hence the method of experiment chiefly adopted here, for which all science-teachers and students are deeply indebted to Professor Tyndall, who carried lantern demonstration to an extent and perfection never before attained. The magnificent apparatus of the Royal Institution, however, appears to have created an impression that electric cameras and other very costly appliances are necessary for effective work of this class; whereas the greater number of experiments can be shown satisfactorily to at least a science class with even a good gas-burner; while any good lantern can be made at small expense a very efficient piece of apparatus. To make this also clear, and thus induce many teachers to substitute the most perfect kind of demonstration for far less striking methods, or for mere diagrams, is a second object of these pages. At the same time I have tried not to forget the private student, and not only to give sufficient hints for him to make most of the experiments without lantern or other bulky apparatus, but especially to find

abundant manual work for him, more particularly in the fascinating domain of polariscope phenomena.

I cannot but hope room may be found for such a work as this, when I recall how much my own delight in the experimental study of Optics has been increased by the individual help and teaching of a few friends. It is never easy, and but seldom possible, to acknowledge all such obligations; but mine have been very special in two directions. To the Rev. Philip R. Sleeman, F.R.A.S., F.R.M.S., I was not only indebted before this little book was thought of, but have been since, for many references to foreign papers and memoirs, and other "detached" information which only his wide and general acquaintance with the whole range of Physical Optics could have supplied. And to Mr. C. J. Fox, F.R.M.S., I owe my entire practical education in that mica-film work, which I hope others may find as attractive as I have done, and which so admirably illustrates the phenomena of polarised light. But for these two friends, some of what are likely to be the most acceptable among the following pages would never have been written.

In regard to the experiments described, there are two things to be said. It would have been desirable, if possible, to have stated the originator of every experiment; but it Attempt has been made to indicate,

was not possible.

as far as known, the first to employ any striking very recent experiment: but many of great beauty seem now such

common property, that it is difficult to ascertain who first made them, or first adapted them for projection. I strongly suspect that we owe to Professor Tyndall many more than it has been possible categorically to ascribe to him; and am the more anxious to state this, because his just claims in higher matters appear to me almost studiously ignored by certain Continental physicists. Some arrangements are, to the best of my belief, original; but none are put forth as such except one or two expressly stated, and it should be perfectly understood that no personal claim is implied regarding any other experiments because no credit is given to some one else: the absence of such credit is simply due to sheer ignorance and the difficulty of acquiring knowledge concerning such matters.

The other remark is, that the order of the experiments differs considerably in some cases from that usually adopted. All that can be said on that point is, that such is the result of considerable reflection, and in the belief that the order chosen is, upon the whole, best adapted to the primary end of assisting vivid conception of the physical realities considered, and the relation of the phenomena to one another. Also, while no attempt is made to arrange the experiments in set "Lectures," the order followed is believed to lend itself best to such a connected course of experimental lectures as a teacher would desire to give to his class, extended or abridged as the case may require. I am not without hope