150 ANNUAL OF SCIENTIFIC DISCOVERT. 



inches to one-half inch, were now held before the eyes, so as to give tlie 

 Mm- liirlit various decrees of convergence and divergence, without in the 

 K-a.-t altering the appearance of the moving bodies; this seemed to indicate 

 that their locality was in the retina or in its immediate neighborhood. A 

 position near the axis of vision was selected, and observed, when it was 

 found that these bodies in traversing this spot always pursued the same di- 

 rection and path, disappearing at the same point; other positions near the 

 axis gave like results. 



This would seem to preclude the possibility of the moving bodies being 

 animalcules swimming in the humor of the eye; the most probable remain- 

 ing supposition is, that they are blood corpuscles circulating in the retina or 

 in its immediate neighborhood. The apparent diameter of these bodies 

 when seen projected on a window six feet distant may be about ^ of an 

 inch, which corresponds to about Tg^y^ of an inch on the retina. The 

 average diameter of the blood globules is ^Vo of an mcn > but taking into 

 account the fact that the shadows of the moving bodies are not well defined, 

 the correspondence may be considered pretty satisfactory. 



The question now arises as to the manner in which the blue glass renders 

 the circulation visible; for these moving shadows cannot be seen with dis- 

 tinctness through red, orange, yellow, green, nor even purple, media; they 

 are, on the other hand, well shown by a certain thickness of a solution of the 

 cupro-sulphate of ammonia. Yellow solutions, when combined with the 

 blue glass or blue solutions, render the circulation invisible, and it does not 

 reappear till the yellow solution has been made so dilute as barely to 

 preserve a yellow tint, and to transmit the spectrum almost unaltered. This 

 shows that the indigo and violet rays are principally concerned in the pro- 

 duction of this appearance; but that it cannot be attributed to fluorescent 

 properties in the blood discs is indicated by the fact that the circulation can 

 be seen through a considerable thickness of crown glass, through an infusion 

 of red sanders Avood mixed with ammonia, as well as through a solution of 

 the bisulphate of quinine. 



The only explanation that has occurred to me as being probable is the 

 following: the blood discs are yellow, and consequently opaque, to a great 

 extent, to the indigo and violet rays ; they would, therefore, in passing before 

 the retina, cast shadows on it; now, the retina being already strongly im- 

 pressed with blue light, that portion of it which was momentarily protected 

 from the action of this light would experience the complementary sensa- 

 tion, or would see, instead of a moving shadow, a yellowish moving streak. 

 This explains, also, why the appearance is not seen with any distinctness in 

 red, orange, yellow, or green light, for yellow media are, to a great extent, 

 transparent to all their rays, and therefore fail to cast shadows. These 

 observations, if new, may be of some interest to those engaged in the study 

 of the physiology of the eye. 



ON OUR INABILITY FROM THE RETINAL IMPRESSION ALONE TO 

 DETERMINE WHICH RETINA IS IMPRESSED. BY PROFESSOR WIL- 

 LIAM B. ROGERS. 



^ Although on first view it might be supposed that an impression made in 

 H-I- eye must necessarily be accompanied by a mental reference to the par- 

 ticular organ impressed, it will be seen from the following simple experi- 



