THE president's ADDRESS. 191 



No authority in these days would doubt that I am entirely 

 right in the particulars on which Sir David took me to task. No 

 point but A could be seen by centrical rays, or on the same part 

 of the retina without the hole as with it. As a preliminary step 

 in establishing an entoptical method, I was proving, both 

 geometrically and experimentally, that in the use, as about to be 

 proposed, of a divergent beam of light, all the rays, save one, 

 traverse the eye eccentrically, and cast entoptical shadows upon 

 the retina accordingly. Brewster does not deign to notice that 

 such a conception is incompatible with his entoptical scheme, but 

 by informing me ex cathedra that they all traverse it centrically, 

 not only hoists his colours, but nails them to the mast. In his 

 recapitulation of 1856, two numerical data that had been omitted 

 in the reprint of 1848 were supplied. I was then enabled to 

 show, without using his letter, that the results in his calculations 

 could only have been obtained on the centrical theory. 



Were the history of entoptical methods amended in agreement 

 with the foregoing statements, it has still another phase of 

 which Helmholtz seems to know nothing. As early as 1834, 

 Capt. Kater (see his letter to Guthrie — in the latter' s work on 

 Cataract) suggested the exploration of the eye by the help of a 

 lens of about 1^ inch focal length, showing how by its means the 

 image of a light, or focus of the convergent beam derived from it, 

 might be carried into the depths of the eye, and its rays crossing 

 one another thereat proceed from that point as a divergent beam, 

 so that these two beams taken together may be conveniently 

 termed a decussant beam ; and inasmuch as the shadows of all 

 intraocular corpuscles in the transit from one portion of the beam 

 to the other become, through the decussation of rays, inverted in 

 attitude and position, we might thus judge of, and approximately 

 calculate their depths in the eye. If there are difficulties in 

 such a mode of examination of which its ingenious author was 

 not aware, it would have been adequate, nevertheless, for the 

 solution of some prominent entoptical problems. The late Dr. 

 Mackenzie, of Glasgow, one of the earliest and most indefa- 

 tigable observers in entoptics, and annotators of its history, 

 adopted this hint (Edinbiu-g Medical and Surgical Journal of 

 Science, July, 1845, a mid-date as to that of my paper and that of 

 Listing's), and even worked with two such beams, obtained by a 

 lens of the requisite focal length from two lights, as Brewster 



