IVOVEMBER 2, 1917] 



SCIENCE 



435 



in four of the books au attempt is made to 

 state a few of the more important conse- 

 quences of the theory, but in practically every 

 case this discussion is limited to one or two 

 paragraphs, either at the very end of the book 

 or at the end of the su.bject of electricity. 

 (It is a striking fact that nearly all of these 

 authors who deem the electromagnetic theory 

 of light worthy of any comment at all, dis- 

 cuss it, not where we would naturally look 

 for it — under the head of " Light " — ^but 

 imder " Electricity," and then proceed calmly 

 to ignore it when "Light" is taken up!) In 

 only one of the test-books examined is there 

 any attempt at the outset to make clear to 

 the student what light really is or to bring 

 out the fact that there is an intimate con- 

 nection between optical and electrical phe- 

 nomena. In not one of the books is the elec- 

 tromagnetic theory made the basis of the 

 treatment of light. 



In most of our test-books there is a chapter 

 entitled "The Nature of Light" or "Theo- 

 ries of Light," in which pains are taken to re- 

 late the triumph of the wave theory of light 

 over the corpuscular theory, but in practically 

 every case the author stops short before com- 

 ing to the crux of the whole matter; there is 

 no suggestion as to what kind of waves light 

 waves are. This is a question which is sure 

 to occur to the student, if he be of a normally 

 inquiring turn of mind, but his perplexity is 

 left unanswered. Certainly no teacher would 

 think of omitting from a discussion of sound 

 waves an explanation of what kind of waves 

 soimd waves are; yet this is the common pro- 

 cedure when light waves are discussed. 



Only two ideas suggest themselves as rea- 

 sons for the common neglect of so important 

 a principle; either the electromagnetic theory 

 is thought to be not yet sufficiently well 

 established to find a place in our text- 

 books, or it is thought to be too difficult for 

 the average student to grasp. As to the for- 

 mer, few will question the fact that the theory 

 has been abundantly verified from every 

 point of view and has been firmly established 

 long enough to justify its occupying a promi- 

 nent place in our text-books and lectures. 



The opinion is widely prevalent, however, 

 that the electromagnetic theory presents diffi- 

 culties so great as to be insuperable for the 

 average college undergraduate. While it may 

 be admitted that the mathematical develop- 

 ment of Maxwell's equations and their appli- 

 cation to the various cases of reflection, re- 

 fraction, and dispersion are decidedly beyond 

 the grasp of the average sophomore, yet it 

 is surely possible to present the essentials of 

 the theory in non-mathematical form, and to 

 discuss its more important consequences, as 

 was attempted by the writer in a recent 

 number of The Scientific Monthly. As to the 

 vagueness which many feel to be inherent in 

 any attempt to picture a light wave on the 

 electromagnetic theory, we may remark that 

 our conception of an electromagnetic wave is 

 precisely as definite as our ideas of an electric 

 or magnetic field. 



It is true that many of the phenomena of 

 light can be given a very simple explanation 

 in terms of the so-called " elastic solid 

 theory," but whatever the advantages offered 

 by the conventional mode of presentation, 

 they are more than counterbalanced by the 

 simple fact that in the light of our present 

 knowledge it is not true to the facts. Cer- 

 tainly our aim in teaching should be to in- 

 culcate a knowledge of reality, not of con- 

 venient fictions with regard to the processes 

 of nature. In more than one of the text- 

 books under consideration frequent reference 

 is made to the " vibrating ether particle " 

 which it is assumed serves to transmit a 

 light wave. It would be interesting to know 

 just what sort of a thing an " ether particle " 

 is conceived to be, but quite apart from the 

 absurdity involved in the use of such a term, 

 there can be no doubt that the conception 

 which the expression " vibrating ether par- 

 ticle " tends to fix in the mind of the student 

 is erroneous and misleading. And so with 

 certain other of the stock phrases we have 

 become accustomed to use in dealing with the 

 phenomena of light. 



The ideal course in optics, in the opinion 

 of the writer, should be based from first to 

 last upon the electromagnetic theory. A 



