COLOR- VISION^ AND COLOR-BLINDNESS.* 



By R. Brudenell Carter. 



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 expan- 

 sion of the eye, of an inverted optical image of external objects — an 

 image precisely analogous to that of the photographic camera. The 

 retina iines 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, 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 

 fiber-layer, mainly composed of the fibers of the optic nerve, which 

 spread out radially from their point of entrance in every direction, 

 except where they curve around the central portion of the membrane; 

 and the perceptive layer, which — as viewed from the interior of the eye- 

 ball, may be likened to an extremely fine mosaic, each individual i)iece 

 of which is in communication with a nerve fiber, by which the impres- 

 sions 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 collecting ajiparatus, calculated togather 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 

 impressions. The distribution of the two elements differs greatly in 

 different animals; and the diiierences point to corresponding differ- 

 ences 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 



* Lecture delivered at tlie lio.val lustitntion ou Friday, May 9, 1890. (From Xature, 

 May 15,1890, vol. xui.pp. 5.=i-61.) 



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