186 THE peesident's address. 



and can offer no guarantee for its exactness. The compilation 

 of a history of physiological optics that might really be relied 

 on would be a task, which would occupy the time and strength 

 of an inquirer for long years. . ." 



This great work is, no doubt, incomparable in its kind, coming 

 as it does from the hand of a master who could say that his 

 chief effort in composing it had been "to convince himself by 

 his own inspection and experience of the justness of all its 

 moderately weighty facts." 



This handbook devotes a chapter, and a supplementary one, 

 to entoptical phenomena. Out of these I mean to reinforce the 

 adage that truth is stranger than fiction ; as well as another that 

 history and fiction are often synonomous. 



Perhaps, on this occasion, a definition might not be unwelcome, 

 thus: — "The light that enters the eye causes us to see, under 

 certain conditions, a series of objects that exist in or on the 

 organ itself. An investigation of these conditions is called 

 Entoptics." Many observers had projected the shadows of such 

 objects upon the retina by means of a beam of divergent light; 

 yet it was left to Sir D. Brewster to conceive the idea of 

 obtaining a pair of such shadows of a given corpuscle by 

 employing a pair of such beams and of determining its distance 

 from the retina by means of the observed angular separation of 

 the shadows. His memoir was read before the Eoyal Society of 

 Edinburgh in 1843, and was thus announced : — " On the Optical 

 Phenomena, Nature, and Locality of Mmcce Volitantes, with 

 Observations on the Structure of the Vitreous Humour, and on 

 the Vision of Objects placed within the Eye." In 1848 it was 

 reprinted, in extenso, in the Philosophical Magazine, of which he 

 was an editor. Again, as late as 1856 in an article on "The 

 Sight and How to See," avowedly by himself (the editor), it 

 was substantially reproduced in the North British Review. 

 There is little to be wondered at in these reassertions on the 

 part of Brewster ; for his solution of the problem continued to 

 be received as authoritative in this country, and had enhanced 

 his reputation among foreigners. 



By general consent he was believed never to have had but two 

 rivals in this research, namely Listing and Bonders. What the 

 former accomplished is detailed in his Beitrag (1845), of which 



