STOKES' "BURNETT LECTURES" 269 



the first two volumes ; and we are now in a position to judge of the work as a 

 whole. But we must first speak of the contents of the present volume. 



The author commences by extending the term "Light" to radiation in 

 general.... 



Next comes a curious suggestion of analogy between the behaviour of fluo- 

 rescent bodies (which always degrade the refrangibility of the light they give off) 

 and the heat-radiation from bodies which have been exposed to sun-light. Sun-light, 

 as it reaches us after passing through the atmosphere, is less rich in ultra-red rays 

 than is the radiation from the majority of terrestrial sources; while the radiation 

 from bodies which have been heated by direct sun-light is entirely ultra-red. Here 

 we have, for the terrestrial atmosphere, the "green-house theory" which, in the 

 second course, was applied to explain some of the singular phenomena exhibited by 

 comets. 



This is followed by an extremely interesting discussion of the functions of the 

 colouring matters of blood and of green leaves : with the contrasted effects, upon 

 plants, of total deprivation of light, and of continuously maintained illumination. 

 A particularly valuable speculation, as to the probable nature of the behaviour of 

 chlorophyll, is unfortunately too long for extraction. 



So far, radiation has been treated without any special reference to vision. But 

 the author proceeds to describe the physical functions and adaptations of the eye : 

 with particular reference to the arrangements for obviating such of the theoretical 

 defects as, while involved in its general plan, would also tend to diminish its practical 

 usefulness. The introduction of this obviously natural proviso, one which we do not 

 recollect having seen prominently put forward till now, exhibits in a quite new 

 light the intrinsic value of those objections to the " argument from design " which 

 have been based upon the alleged imperfection of the eye as an optical instrument. 



The analogy of fluorescence is once more introduced, but now for the purpose 

 of suggesting a mechanical explanation of the mode in which the sense of vision is 

 produced. This is brought forward after the modern photo-chemical theory of vision 



has been discussed The triplicity of the colour-sense, and the mechanism of single 



vision with two eyes, are treated at some length. But throughout this part of the 

 work it is frankly confessed that there are many elementary questions, some of funda- 

 mental importance, which we are still unable even approximately to answer 



No higher praise need be bestowed on the scientific part of this third volume 

 than is involved in saying that it is a worthy successor to the other two. Together, 

 they form a singularly instructive, and yet (in the best sense) popular, treatise on a 

 fascinating branch of natural philosophy. Were this their only aim, no one could 

 deny that it has been thoroughly attained. 



But their aim is of a loftier character. Here and there throughout the work 

 there have been occasional references to the main purpose which has determined 

 the author's mode of arranging his facts and his deductions from them. In the few 

 closing pages this purpose is fully developed, and a brief but exceedingly clear 

 statement shows at once how much in one sense, and yet how little in another, can 

 be gathered as to the personality and the character of the Creator from a close and 

 reverent study of His works. 



