373 



satisfactory to those who arranged the apparatus, is open to the ob- 

 jection, on the part of others, that the picture does not present any 

 prima facie evidence of its being the result of rays which passed 

 through the eye. We therefore endeavoured to copy photographi- 

 cally the actual image which is depicted on the retina. To do so, 

 another bullock's eye was carefully dissected, so as to open a circular 

 space of about three-eighths of an inch in diameter at the back of 

 the eye, the retina was removed, and a very thin film of glass, in 

 shape like a watch-glass, substituted for it; this supported the vi- 

 treous humour in its original position, and served also to prevent its 

 contact with the photographic paper placed behind to receive the im- 

 pression. In another trial, the retina was left untouched, without 

 altering the ultimate result. 



" Iodide of silver paper was then made sensitive to light by a wash 

 of gallo-nitrate of silver, and used as in the Talbotype process, small 

 squares of the wet paper being successively applied to the back of 

 the thin glass film, and exposed for varying periods (one minute on 

 an average) to the different objects to which the bullock's eye was 

 presented. On developing the latent images with strong gallo- 

 nitrate of silver, very distinct pictures were obtained of a key and 

 of a spotted window curtain. These negative pictures are inclosed. 

 It is thus beyond a doubt that the chemical rays penetrate the hu- 

 mours of the eye, and impinge upon the retina. 



" Allan* B Dick. 

 " John Spiller." 



It thus appears that the actinic or chemiral rays are not arrested 

 in their passage across the chamber of the eye ; and it becomes an 

 important question how they will affect the general surface of the 

 retina on which they impinge, and what share they have in pro- 

 ducing vision. Into this problem, as a whole, however, I do not 

 purpose to enter : the question I alone consider is the change which 

 the actinic rays will undergo when they fall upon that peculiarly or- 

 ganized portion of the human retina which anatomists distinguish as 

 the " yellow spot." This " spot," almost peculiar to man, presents 

 a diameter of about -^^(h. inch, and occupies the bottom of the eye, 

 in the exact axis of its transparent humours. It is more transparent 

 than the rest of ihe retina, and has long been recognized as the seat 

 of most perfect vision in the eye of man. I have elsewhere drawn 

 VOL. III. 2 I 



