546 



NATURE 



{April lo, il 



would be obliged to have a diameter of rather more than 

 four miles, say four miles, and the distances from the 

 source of sound to the hole through which it passes, and 

 from that again to the place where the sound is listened 

 to, would have to be 4000 miles each. 



" It is remarkable that the existence of rays, which 

 forme 1 the great stumbling-block in the way of the 

 early reception of the theory of undulations, is now shown 

 to b;long to a class of phenomena, those of diffraction, 

 the complete and marvellously simple explanation of 

 which afforded by the theory of undulations now forms 

 one of the great strongholds of that theory." 



In connection with the Lecture on the Senses, by Sir 

 W. Thomson, which has re:ently appeared in Nature 

 (vol. xxix. pp. 438, 462) we may t >ke the following pas- 

 sage. [At the same time it may be well to remark, in 

 passing, that Sir W. Thomson omits altogether the Sense 

 of Rotation, which seems to be fully established by the 

 researches of Crum Brown, De Cyon, Fiourens, Mach, 

 &c. He also distinguishes between the Senses of Touch 

 and of Heat, making the so-called Muscular Sense a case 

 of the former ; while it seems more probable that Touch 

 and Heat are the same sense, and the Muscular sense an 

 independent one.] 



" As regards the mode of perception, while there are 

 analogies between sound and light there are at the same 

 time notable differences. In sound, the tympanum of the 

 ear is thrown mechanically into vibration, and the nerves 

 of hearing are mechanically afiected, as a mechanical 

 disturbance of a point on the surface of the body is made 

 known by the sense of touch. But in light, just as we 

 have seen reason to believe that it is the disturbance of 

 the ultimate molecules, or of their constituent parts, by 

 which the vibratory motion which constitutes light is in 

 the first instance communicated from ponderable matter 

 to the ether, so we have reason to think that when light 

 is absorbed what takes place is that the disturbance of 

 the ether is communicated, not to portions of matter re- 

 garded as forming portions of a continuous elastic body, 

 but to the ultimate molecules of which matter consists, 

 or to their constituent parts. It may be that temporary 

 chemical changes are thereby produced in the ultimate 

 filaments of the nerves of the retina, in which case the 

 sense of sight would be more analogous to the sense of 

 taste than to that of touch." 



As a specimen of the firm, yet cautious, way in which 

 the Lecturer meets the grand difficulty of his position, 

 take the following : — 



" In studying this subject, one can hardly fail to be 

 struck with the combination of these two things : — the 

 importance of the ends, the simplicity of the means. 

 When I say the importance of the ends, I use a form of 

 expression which is commonly employed as expressing 

 design. And yet on that very account we must be on our 

 guard against too narrow a view. When we consider the 

 subject of vision in its entirety, the construction of the 

 recipient organ as well as the properties of the external 

 agent which affects it, the evidence of design is such, it 

 seems to me, as must to most minds be irresistible. Yet 

 if I may judge of other men's minds by my own, it is 

 rather in the construction of the recipient organ than in 

 the properties of the agent that affects it that the evidence 

 of design is so strongly perceived. And the reason of 

 this may be that we are here dealing with what more 

 nearly resembles design as we know it in ourselves. Man 

 takes the laws of matter as he finds them ; the laws of 

 cohesion, of the conversion of liquid into vapour, of the 

 elasticity of gases and vapours, and so forth ; and in sub- 

 serviency to those laws he constructs a machine, a steam- 

 engine for instance, or whatever it may be ; but over the 



laws themselves he has absolutely no control. Now when 

 we contemplate the structure of the eye we think of it as 

 an organ performing its functions in subserviency to laws 

 definitely laid down, relating to the agent that acts upon 

 it, laws which are not to be interfered with. We can, it is 

 true, go but a little way towards explaining how it is that 

 through the intervention of the eye the external agent 

 acts upon the mind. Still, there are some steps of the 

 process which we are able to follow, and these are 

 sufficient to impress us strongly with the iiea of design. 

 The eye is a highly specialised organ, admirably adapted 

 for the important function which it fulfils, but, so far as 

 we can see, of no other use ; and this very specialisation 

 tends to make the evidence of design simpler and more 

 apparent. But when we come to the properties of the 

 external agent which affects the eye, we begin to get out 

 of our depth. These more nearly resemble those ulti- 

 mate laws of matter over which man has no control ; and 

 to say that they were designed for certain important 

 objects which we perceive to be accomplished in sub- 

 serviency to them, seems to savour of presumption. It is 

 but a limited insight that we can get into the system of 

 nature ; and to take the very case of the luminiferous 

 ether, while as its name implies it is all-important as 

 regards vision, the present state of science enables us to 

 say that it serves for one object of still more vital im- 

 portance ; we seem to touch upon another ; and there 

 may be others again of which we have no idea." 



At the end of the work we arc told that the two volumes, 

 which are to follow this, are to deal with 



I I. Researches in wldch Light has been used as a means 

 of imwstigation, and 



III. Light, considered in relation to its beneficial effects. 

 The former of these we may hope to have in a year 



from the present time ; for the final volume we must wait 

 a year longer. But in the meantime let us be thankful 

 for the first instalment, which is a masterpiece of sim- 

 plicity and strength ; and be grateful to the Commis- 

 sion, and the Trustees, to whom we are so very directly 

 indebted for it. And, above all, let us lay to heart the 

 valuable lesson which the Author has drawn from the 

 story of the two rival theories of Light, and of their chief 

 supporters, a lesson good for all time : — 



" It may be said. If the former theory is nowadays ex- 

 ploded, why dwell on it at all .' Yet surely the subject is 

 of more than purely historical interest. It teaches lessons 

 for our future guidance in the pursuit of truth. It shows 

 that we are not to expect to evolve the system of nature 

 out of the depths of our inner consciousness, but to follow 

 the painstaking inductive method of studying the pheno- 

 mena presented to us, and be content gradually to learn 

 new laws and properties of natural objects. It shows 

 that we are not to be disheartened by some preliminary 

 difficulties from giving a patient hearing to a hypothesis 

 of fair promise, assuming of course that those difficulties 

 are not of the nature of contradictions between theresults 

 of observation or experiment and conclusions certainly 

 deducible from the hypothesis on trial. It shows that vv'e 

 are not to attach undue importance to great names, but to 

 investigate in an unbiased manner the facts which lie 

 open to our examination.'' 



On this it would be impertinent to make any farther 

 comment. P. G. Tait 



OUR BOOK SHELF 



Absolute Measnreinents in Electricity and Magnetism. 



By Andrew Gray, M.A., F.R.S.E. (London : Mac- 



millan and Co., 1SS4). 

 This book, which is mainly a reprint of a series of papers 

 on absolute measureme'n of electric currents and poten- 



