116 Mr. B. Brudenell Carter [May 9, 



WEEKLY EVENING MEETING, 

 Friday, May 9, 1890. 



Sir James Ceichton Browne, M.D. LL.D. F.E.S. Treasurer and 

 Vice-President, in tlie Chair. 



R. Brudenell Carter, Esq. F.R.C.S. 



Colour-Vision and Colour-Blindness. 



It is a matter of familiar knowledge that the sense of vision is called 

 into activity by the formation, on the retina or internal nervous 

 expansion of the eye, of an inverted optical image of external 

 objects — an image precisely analogous to that of the photographic 

 camera. The retina lines the interior of the eyeball over somewhat 

 more than its posterior hemisphere. It is a very delicate transparent 

 membrane, about one-fifth of a millimetre in thickness at its thickest 

 part, near the entrance of the optic nerve, and it gradually diminishes 

 to less than half that thickness at its periphery. It is resolvable by 

 the microscope into ten layers [shown], which are united together by a 

 web of connective tissue, which also carries blood-vessels to minister 

 to the maintenance of the structure. I need only refer to two of these 

 layers ; the anterior or fibre-layer, mainly composed of the fibres of 

 the optic nerve, which spread out radially from their point of entrance 

 in every direction, except where they curve around the central por- 

 tion of the membrane ; and the perceptive layer, which, as viewed 

 from the interior of the eyeball, may be likened to an extremely fine 

 mosaic, each individual piece of which is in communication with a 

 nerve fibre, by which the impressions made upon it are conducted to 

 the brain. The terminals of the perceptive layer are of two kinds, 

 called respectively rods and cones ; the former, as the name implies, 

 being cylindrical in shape, and the latter conical. The bases of the 

 cones are directed towards the interior of the eye, so as to receive the 

 light ; and it is probable that each cone may be regarded as a col- 

 lecting apparatus, calculated to gather together the light which it 

 receives, and to concentrate this light upon its deeper and more 

 slender portion, or posterior limb, which is believed to be the portion 

 of the whole structure which is really sensitive to luminous impres- 

 sions. The distribution of the two elements diff'ers greatly in different 

 animals; and the differences point to corresponding differences in 

 function. The cones are more sensitive than the rods, and minister to a 

 higher acuteness of vision. In the human eye, there is a small central 

 region in which the perceptive layer consists of cones only, a region 

 which the fibres avoid by curving round it, and in which the other 

 layers of the retina are much thinner than elsewhere, so as to leave 

 a depression, and are stained of a lemon-yellow colour [shown]. In a 



