188 THE president's address. 



it, that is to say, not that he essentially deviated from it or 

 impugned its accuracy any more than Listing had. 



It was in 1856, the very year in which Brewster recapitu.lated 

 his achievements in the North British Review, as if totally 

 unconscious of what Listing and Donders had written that 

 Helmholtz made this award. 



Notwithstanding the seeming invincibility of Brewster's 

 position, it was destined to be seriously assailed in 1864. In 

 that year a volume on the "Anomalies of Accommodation, &c., 

 of the Eye," from the pen of Donders, was published, in which 

 there is given a short "History of Entoptic Observation." In 

 it Brewster's absolute priority is granted, also that he was the 

 first to double the shadows, and that he " even made a calculation 

 of the position of one of his tmiscte volitantes.^'' Nevertheless, 

 after passing on to commend Listing's method as fully developing 

 the theory, in introducing a description of his own modification 

 of it, Donders remarks ' ' that of Brewster presented difficulties 

 in the projection, and the calculation was imcertain and trouble- 

 some. . ." "Whilst such a cardinal point as that the said calcu- 

 lation was actually effected by Brewster's method is admitted, I 

 cannot pretend to seize the import of the exceptions Donders 

 would take. But if it could be made to appear that in his earlier 

 essays he had raised explicit exceptions, that might make any 

 of my comments superfluous, the fact that they should have had 

 no influence on Helmholtz' history, would only add force to the 

 tenour of my argument. 



However, whilst this work of Donders was in the press, my 

 own Entoptics was in the same predicament. This also contained 

 a history of entoptic methods, the avowed purpose of which was 

 to correct and expand that furnished by Helmholtz. In this, I 

 undertook to demonstrate from Brewster's own papers that all 

 his remarks and all his calculation involved the false assumption 

 that all shadows of intraocular objects, observed in his divergent 

 beams of light, were cast upon the retina by rays that passed 

 through the lenticular (or optic) centre of the eye : that is, that 

 he had never emancipated himself from an old misconception of 

 De la Hire's, which he cites in the beginning of his paper. 

 Independently of which, as I have also pointed out, his calcu- 

 lation of the retinal distance of a musca rests upon another 

 grave optical error, that in itself would vitiate the result. Hence 



