3 46 Mr. T. Smith's Investigation of certain Phenomena of 



the chromatic aberration ; for every white image falling on the 

 retina before or behind its true focal point is surrounded by a 

 1-ed or xnolet border, which, as well as the other cause, inter- 

 feres with its distinctness. Now it is certainly a very extra- 

 ordinary circumstance, that diminished sensibility to red light 

 around the white image should occur then, and tiien only, 

 when it is surrounded by this red or violet border. A distinct 

 red border around a distinct bright object produces no such 

 effects, as I have proved by experiments carefully made : it 

 follows, therefore, incontrovertibly, that it is not the physical 

 difference between a distinct and an indistinct white image 

 that excites those changes in the sensibiHty which have been 

 proved to occur. In regard to a primary coloured image, the 

 difference between it when distinct and when indistinct, consists 

 in that diffusion and mixing of the rays in the latter which has 

 been noticed above, and which not only obscure the outline, 

 but the whole surface. If these scattered rays, therefore, pro- 

 duce the changes in the sensibility that take place in these 

 circumstances, we must be compelled to acknowledge that the 

 same physical cause produces directly contrary effects at the 

 same time ; for, if this be true, the scattered rays that obscure 

 the surface of the primary coloured image, increase the sen- 

 sibility, in a remarkable degree, to the same kind of light, and 

 the scattered rays that obscure the outline, diminish, in the 

 same degree, the sensibility to the same kind of light. The 

 supposition is manifestly absurd, and therefore we I'eturn with 

 increased confidence to our first conclusions, that indistinctness 

 of a 'white image is the true exciting cause of the diminished 

 sensibility to red light that takes place around it in the exposed 

 eye, and of the increased sensibility to red light that occurs in 

 the other eye at the same time ; and that indistinctness of a 

 primary coloured image is the real exciting cause of the in- 

 creased sensibility to that colour which ensues to the image 

 itself, and the diminished sensibility to the same colour that 

 occurs for some considerable space around the image. 



The nature of the effects and the true exciting causes of 

 these remarkable affections of sight being ascertained, it only 

 remains to investigate the seat and nature of the actions ex- 

 cited by the indistinctness of the image on the retina. 



In the first place, then, the i-etina, though it has been cus- 

 tomary to consider it as the seat of any changes in the sensi- 

 bility to light, cannot, in these cases, be regarded as the seat 

 of either of these affections in the exposed, eye ; for it is incon- 

 ceivable that undulatory motions, extending from the part of 

 the retina on which the bright light falls, to all parts around 

 it, cau be produced by an indistinct image, when a distinct 



