THE president's ADDRESS. 187 



I possess a copy. The latter's investigations began in 1846, 

 and in 1854 were narrated in full in a latin thesis written by 

 his pupil Doncan, under his own supervision, entitled (plainly 

 after Brewster's paper) Be Corporis Vitrei Structura ; an order 

 for which, I regret to say, failed to procure me a copy, so that I 

 have been obliged to accept Helmholtz's version of its contents. 

 These two essays are the only ones on entoptical methods 

 asterisked by Helmholtz as having been read "with his own 

 eyes." I do not know whether this may account for the 

 strange slij) — for a 3 may readily be mistaken for a 5 — by 

 which he postdates Brewster's paper to 1845, and catalogues 

 it actually underneath Listing's, though its full title is quoted 

 in the latter's Beitrag itself. Wherein attention also is directed 

 to a mention of it in a Grerman Lehrbuch ; suggesting the query, 

 whether Listing himself had ever read it in the original 

 English? 



It was to the musG<2 or flitting corpuscles in the vitreous fluid 

 that is lodged immediately in front of the retina that Brewster 

 more particularly applied his scheme. Listing explored the 

 solid media in the interior part of the eye, by means of a single 

 divergent beam of light supplemented by a rotation of the 

 eyeball, and by thus comparing the movement of the shadow 

 of any corpuscle fixed in either of them with that of the shadow 

 of the iris — not troubling himself with actual numerical calcu- 

 lations. Bonders adopts a couple of divergent beams upon the 

 eye at rest, in such a way as to compare the distance of a musca 

 from the retina with that of the iris, by observing the angular 

 distances between their repective pairs of shadows. 



Helmholtz having noticed the casual remarks of early observers, 

 sums up the claims of these three originators, thus: — "As to a 

 more stringent theory of the phenomena, the methods for judging 

 of the places of the corpuscles in the eye were first of all estab- 

 lished much more lately by Listing and Brewster, whom more 

 lately Bonders followed." The second is here shorn of his 

 absolute priority ; but elsewhere, in speaking of the vitreous 

 humour, an abstract of his method is given by the historian, 

 and its fitness for calculating the distance of any corpuscle 

 floating in it from the retina corroborated. It being appended : — 

 " Bonders has altered this method." Altered so as to simplify 



