266 



NATURE 



[July 21, 1892 



we have ourselves been led, though there are not a few 

 matters on which we have the pleasure of agreeing with 

 him ; his psychological and philosophical views are not 

 in all respects those which we have reached, though 

 here again we are on many not unimportant questions 

 on his side ; but we believe him to be an honest and 

 fearless inquirer after that Truth which stands on the 

 title-page of a work to which, perhaps, for some of our 

 readers, these volumes of essays may form a suitable 

 introduction. C. Ll, M. 



PHYSICAL OPTICS. 

 A Treatise on Physical Optics. By A. B. Basset, 

 M.A., F.R.S. (Cambridge : Deighton, Bell, and Co., 

 1892.) 

 A NEW treatise on the higher branches of physical 

 X 1. optics must be welcome to all who are interested 

 in the subject. Mr. Basset explains in the preface the 

 scops and aim of his book, and it is needless to say that 

 he performs the task he has set himself with ability 

 and success. If, nevertheless, we close the book with a 

 feeling of disappointment, it is because we could have 

 wished that the author had been more ambitious, and 

 attempted to give us a little more than a compilation 

 of the standard papers on the subject. There is one 

 sentence in the preface which, though it evidently does 

 not express what the author meant to say, yet may serve 

 as a peg whereon to hang the only criticism which can 

 fairly be raised against Mr. Basset's treatment of his 

 subject. " I have a profound distrust," says the author, 

 "of vague and obscure arguments based upon general 

 reasoning instead" of upon rigorous mathematical 

 analysis." Now, if we are to have vague and obscure 

 arguments, it does not seem to matter much whether 

 they are founded upon general reasoning or upon 

 mathematical analysis, however rigorous that may be. 

 In a subject which is in a state of growth, it may be 

 possible to hide, but it is impossible to avoid, all obscurity 

 and vagueness ; and original work ever consists in the 

 attempt to overcome such obscurities. By purposely 

 excluding everything that is vague from a physical 

 treatise, we destroy all possibility of making the work 

 useful in stimulating furthei research. There are two 

 ways of dealing with difficulties : we may try to 

 overcome them, or we may run away from them. Mr. 

 Basset chooses the latter course, and though some 

 of us might have wished him to be a little more 

 venturesome, we gratefully accept what he has given us, 

 and the above remarks only apply to certain parts of the 

 book. After an introductory chapter, Mr. Basset treats 

 of the interference of light. He follows the time-honoured 

 custom of taking Fresnel's mirrors and the biprism as the 

 simplest case of interference. The effects which are 

 observed are seriously modified, however, by so-called 

 diffraction effects, and we might perhaps have expected a 

 book of this kind to have entered a little more fully into 

 the subject. That the author avoids all reference to 

 experimental details is a distinct advantage, and 

 renders his book more lucid and valuable for reference. 

 It is much to be wished the author's plan could be 

 more generally followed, and that all lengthy discussions 

 NO. I 186, VOL. 46] 



of instrumental details could be kept out of theoretical 

 treatises, and relegated to separate books. 



The diffraction of light is fully discussed in chapters 

 iv., v., and xiii. Mr. Basset has followed safe guides in 

 the treatment of his subject, and it is perhaps this part of 

 the book which will be specially valuable to the teacher 

 and student. 



It is well that the phenomena of double refraction 

 should be first approached without more allusion to the 

 difficult subject of the constitution of the ether than is 

 absolutely necessary, and this is perhaps most easily done 

 by following, as Mr. Basset does, the historical method, 

 and starting from Fresnel's deductions. 



The colours of crystalline plates are, of course, treated 

 in an important chapter, and it is worthy of note that 

 Mr. Basset does not introduce the somewhat misleading 

 distinction between the effects produced by parallel 

 and by convergent or divergent beams. In the usual 

 polariscopes a number of parallel beams pass through 

 the crystalline p!ate in different directions. If the 

 optical arrangement between the plate and the eye is 

 such that these various beams enter the eye, we get the 

 phenomena which are often called interference effects in 

 divergent light, while if those beams only which make 

 a small angle with each other are allowed to pass the 

 pupil, we get the uniform tint described as the effect of 

 parallel light. Both kinds of effects might also be pro- 

 duced if instead of parallel beams we had a number of 

 pencils diverging from points in a plane close to the 

 crystalline plate. In either case the eye is supposed to 

 focus for an infinite distance, and the different appear- 

 ance is only one of degree, depending on the extent of 

 the angle between the different rays passing through the 

 crystal and into the eye. 



Mr. Basset enters fully into the consequences of the 

 various hypotheses which have been made as regards the 

 differences of density or elasticity of the ether in different 

 media. The investigations referred to by him are, of course, 

 of the utmost importance, but it should have been pointed 

 out that as regards application to optics they are 

 wanting in reality. We know enough now to be 

 able to say that the medium does not behave like an 

 elastic body, and in some form or other the electro-mag- 

 netic theory must be considered as established. It seems 

 idle, therefore, to discuss whether the hypothesis of Green 

 or of Neumann is most contradicted by experiment. It 

 would have been perhaps worth while to bring out more 

 clearly the fact that no elastic theory of the ether has yet 

 been found satisfactory, and that if the electro-magnetic 

 theory had not come to help us we should be in a 

 very serious difficulty. 



It is true, of course, that we are at present unable, and 

 probably always shall remain unable, to discard the elastic 

 theories, because the study of transverse vibrations can 

 only be satisfactorily carried out with the help of examples 

 in which we understand to some extent the mechanism 

 by which the vibrations are propagated. But unless a 

 writer chooses to follow a purely historical treatment, it 

 would seem to be more satisfactory to separate completely 

 the mathematical study of vibrations from the subject 

 of optics. Treated purely as elastic vibrations we may 

 usefully discuss what would happen at the boundary be- 

 tween two media having different elasticities or densities ; 



