254 S. B. VINCENT 



accompanied by a low standard of acuteness of vision as in 

 ungulates and rodents." 



Popular belief as to the significance of the rods and cones 

 seems to ascribe to the one the specific function of vision in a 

 dim light, colorless vision, and to the other in some hypothetical 

 way color vision. The facts of peripheral vision, color blindness 

 and night vision are brought to the support of this theory. More 

 recent work seems to be greatly modifying the old conceptions 

 of retinal structure and making the distinctions between rods 

 and cones less absolute. The reader is referred to the studies 

 of Hess, 18 Cajal, 17 Bernard, 18 and Cameron. 19 



When the figures giving the refractive indices for lens and 

 eye media are noted it will be seen that there can be little power 

 of accommodation in most of these forms where the refractive 

 indices so nearly approach each other. The figures for man are 

 lens (total refractive index) 1.437, fluid media, 1.336. In fishes 

 the accommodation is brought about by altering the distance 

 between the lens and retina. The rat has no known mechanism 

 for any such purpose nor has any such alteration been observed. 



In the light of this discussion what shall we conclude about 

 the rat's eye. As arguing against its good vision we have a 

 thread-like optic nerve, opaque nerve fibres, no sensitive area, 

 no fovea, no eye movements demonstrable, 20 a pupil which 

 reacts very sluggishly to light, a divergence of the optical axes 

 of 60 to 84 degrees, and in the wild state extreme hypermetropia. 

 That there may be an occasional variant from this last under 

 conditions of domestication is true, but the great curvature of 

 the cornea and sphericity of the lens are indications against its 

 being of frequent occurrence. 



And what shall we say as to the conduct of experimentation? 

 First, we must establish for all experimentation employing vis- 



16 Hess, R. TJeber der Bau und Zapfen der Wirbeltiere. Deutsch zool. Ges., 

 1903, Bd. 13, S. 33-41. See also numerous papers of C. Hess. 



17 Cajal, Ramon y. Histologic du Systeme Nerveaux de l'Homme et des Vert£- 

 bres, Fr. ed., 1911*. 



18 Bernard, Henry M. Studies in the Retina. Quarterly Jour. Mikro. Sc. N. S., 

 1903, Vol. 46, pp. 25-75; Vol. 47, p. 303. 



19 Cameron, John. Farther Researches on the Rods and Cones of the Retina. 

 Jour. Anat. and Physiol., 1911, Vol. 46, pp. 45-53. 



20 1 have never seen a rat's eye move under normal conditions. A very slight 

 nystagmus may be produced by rotation showing that there is some power of 

 movement. 



