i6 



NA TURE 



[February i, 1906 



satisfactorily explained by the innate adaptation of 

 their sensitive cognition and appetite, whereas the 

 hypothesis of animal intelligence is unable to offer 

 any solution. " 



" Instinctive sagacity " seems to me, I confess, a 

 contradiction in terms. 



I admit that the subject is one of much difficulty, 

 but if an ant applied Father Wassmann's rigorous 

 criticism to man himself, 1 am not sure that our 

 boasted gift of reason could be absolutely proved. 



No doubt animals do stupid things, but so do we. 



Father Wassmann describes what he justly calls 

 the " lovely scenes " in an ant's nest— the care of the 

 young, the " motherly tenderness " shown to the 

 delicate pupa; — but denies that this is any evidence 

 of affection, and contrasts it with the love of a woman 

 or a man for their children. This, he maintains, 

 " is a rational love, conscious <</ duty (the italics are 

 his), therefore it is the highest and noblest love exist- 

 ing in Nature." Far be it from me to say a word 

 against either reason or duty. They are amongst the 

 highest qualities of our nature; but surely they have 

 nothing to do with the love we feel for our children, 

 which rests on even nobler feelings. 



While fully recognising, then, the accuracy and 

 interest of Father Wassmann's observations, and after 

 carefully considering his arguments, I cannot but 

 recognise in animals some vestiges and glimmerings 

 of intelligence, and maintain, as I did thirty years 

 ago, that " when we see an ant-hill, tenanted by- 

 thousands of industrious inhabitants, excavating 

 chambers, forming tunnels, making roads, guarding 

 their home, gathering food, feeding the young, tend- 

 ing their domestic animals — each one fulfilling its 

 duties industriously, and without confusion — it is 

 difficult altogether to deny to them the gift of reason ; 

 and the preceding observations tend to confirm the 

 opinion that their mental powers differ from those 

 ot men, not so much in kind as in degree." 



AVEBURY. 



MAXWELL'S THEORY OF LIGHT. 

 The Electromagnetic Theory of Light. By Dr. C. E. 

 Curry. Part i. Pp. xy + 400. (London: Mac- 

 millan and Co., Ltd., 1905.) Price 12s. net. 

 TAR. CURRY bases his work, which is almost 

 *-J entirely analytical, on Maxwell's equations of 

 the electromagnetic field. These equations suffice to 

 account for the phenomena of electromagnetism, and 

 the book is a discussion of the properties of electro- 

 magnetic waves in which the condition that the wave- 

 length is short is generally, but by no means always, 

 introduced. In these equations four vectors are con- 

 cerned, the electric and magnetic forces, and the 

 electric and magnetic displacements, or, as Dr. Curry 

 prefers to call them, the electric and magnetic 

 moments. The type of equation satisfied by each of 

 these vectors is the same, and it is not necessary for 

 Dr. Curry's purpose to identify the light vector de- 

 finitely with either. It is another vector satisfying 

 an equation of the same type. 



No attempt is made to give a mechanical account 

 NO. 1892, VOL. 73] 



of the properties of the ether; it is a medium in which 

 transverse waves of electric and magnetic force are 

 propagated according to the laws indicated by Max- 

 well's equations; in a crystal, however, of course the 

 direi tion of the electric force does not lie in the wave- 

 front ; the same is true of the magnetic force if the 

 permeability be a function of the direction. 



Working on these lines. Dr. Curry has put together 

 a large amount of information as to the analytical 

 properties of such waves. The earlier chapters are 

 entirely taken up with the discussion of the forms 

 defined by certain particular solutions of the equations 

 of motion, for if <t> =f(r + vt) j r be a solution, so is 

 </"(/) 1/1 '•{/yi'iter, where n = A + n + v. Some of the 

 solutions thus obtained are of importance in the 

 theor} of light, but, as the author slates, their interest 

 is chiefly theoretical; and one of his "chief reasons 

 for the elaborate treatment of this particular class of 

 waxes has been to indicate another fertile field of 

 research offered by Maxwell's equations." 



In chapter iv. we an- introduced to the phenomena 

 ..I interference, treated at first in a simple manner, 

 but applied later to the various' kinds of waves the 

 properties of which have already been discussed. The 

 more usual problems of optics first become prominent 

 in chapter v., which deals in the ordinary way with 

 Huyghens's principle and its application to the recti- 

 linear propagation of light. The first difficulty occurs 

 in the attempt to find an expression for the secondary 

 disturbance transmitted from a given element of a 

 primary wave. Such expression may clearly involve 

 the angle </> between the normal to the wave and the 

 direction in which the secondary disturbance is being 

 estimated, but the statement that " it is natural to 

 assume that the law of variation of the light vector 

 . . . be according to the cosine of the obliquity of 

 (he angle <t> " is not very convincing, and there seems 

 no reason lor calling this law the " natural law of 

 obliquity." The law is, of course, a simple one, and 

 it allows of the analytical solution of various problems 

 which are hardly tractable when a more complex law- 

 is assumed; but this is its sole merit. Stokes showed 

 that the true factor is (i+cos^), and this law is 

 utilised later on ; but the physical reason for the 

 change of phase in consequence of which the secon- 

 dary disturbance from a wave sin fefrr— r) becomes 

 proportional to cos k(vt — r) is not discussed as fully 

 as its importance deserves. On these points, reference 

 might with advantage have been made to Prof. 

 Schuster's article in the- Philosophical Magazine, 

 vol. xxxi. — it is quoted later on another point — or to 

 Lord Rayleigh's article in the " Encyclopaedia 

 Britannica." Following this a rigorous proof of 

 Huyghens's principle is given in the usual way from 

 the consideration of the relations existing between 

 certain volume and surface integrals, and the n suit 

 is applied to optical problems; but the fact thai this 

 rigorous analysis leads to Stokes's law of obliquity 

 is not definitely stated, though it follows at once from 

 the formula; on p. 17b. 



Diffraction phenomena are explained by the use of 

 the same principles, employing the most general 

 formula for the secondary disturbance, and assuming 



