XXVI SCIENCE PROGRESS. 



The title of the work is certainly happier than the title of the 

 lecture on which it is founded. It is quite true that both words "sight" 

 and "eyes" are only sharply definable when used of human sight and 

 eyes, and that, when the lower forms of organs for appreciating variations 

 in light intensity are brought into line, the terms are often of more than 

 doubtful applicability. But we do not think that this uncertainty as to 

 what the comparative anatomist and physiologist, as apart from the 

 human anatomist and physiologist, means by sight and eyes justifies 

 the author in applying either term to the sensitiveness of certain skins 

 to light. This, indeed, the author recognises, for on page i8 he modifies 

 the word ScJien into Lichtsinn, and rightly adopts this modification for 

 the title of his book. 



Most people would be inclined to affirm that as the word "sight" 

 was already in use, its meaning was fixed before we had any knowledge 

 of the simpler eyes of the invertebrates, and that, while there can be 

 no objection to calling all organs for the appreciation of light sensations 

 "eyes," we cannot say that all the eyes see unless, from their possession 

 of a dioptric apparatus or otherwise, we can conclude that they are capable 

 of appreciating external form-differences. 



The author writes as an authority, and all that is at present known 

 of the remarkable sensitiveness to light possessed by the skins of many 

 lower invertebrates (the list of which, by the way, he has not exhausted) 

 he discusses with knowledge and ability. His treatise makes it perfectly 

 clear that we are just as much in the dark as to the real causes of these 

 diffuse light sensations of certain skins as we are of the true causes of 

 the light sensation of our own specialised organ of vision. It is in- 

 teresting to note that many of the surfaces sensitive to light, e.g., the 

 siphons of Psammobia, the skin of a "decapitated" Amphioxus, are abso- 

 lutely free from pigment, but nevertheless the author admits that 

 pigment must have some definite connection with sight, inasmuch as 

 it is always present in specialised visual organs, except in albinoes, 

 which are abnormal (" Missbildungen "). As to what this connection 

 is, the author merel}' repeats the old suggestion that it prevents diffusion 

 of the light rays by sheathing the visual cells, in order that they may 

 clearly distinguish separate points of light. On the other hand, it may 

 be pointed out that it is also always present in eyes which do not "see" 

 in the restricted sense above sketched, and which do not require to 

 distinguish separate points of light. 



Cellulose. An outline of the chemistry of the structural elements of 

 plants, with reference to their natural history and industrial uses. 

 By Cross and Bevan. Longmans. 



An almost inevitable result of the increasing activity of scientific 

 investigators of the present day, and the consequent accumulation of 

 scientific literature in almost every department, is the need for the 

 appearance of definite monographs, in which, by specially qualified 

 hands, the vast amount of crude material scattered here and there may 



