230 HENRY LAURENS AND S. R. DETWILER 
The shape of the pupil is a subject of interest in a paper deahng 
primarily with the eye of the alligator because of the vertical 
slit form of the pupil in this animal. In two recent articles 
reference is made to this subject. Rochon-Duvigneaud ('17), 
in listing the characteristics of the eye of the geckos which make 
them adapted to nocturnal vision, includes the form of the 
pupil — a vertical slit — which shows a rapidity of movement 
surpassing that of the human pupil and approaching that of 
birds. In dim light the pupil is a large oval, or even round, as 
in the cat. In bright light it is closed completely. According 
to Rochon-Duvigneaud, a round pupil can dilate as well as an 
oval one, but it cannot be entirely closed, and he beheves that 
it is in the way of a protection against an excess of light in an 
animal adapted to twilight vision that an oval pupil finds its 
chief function. It is possible to imagine that a pupil in the form 
of a vertical sht can be opened wider than if it were round (for 
example, the cat) . 
Hartridge ('19) views the function of a vertical sht pupil (as 
seen in the cat) from another angle, viz., the function of the lens 
and the aberrations caused thereby, and the habits of life of 
the cat family in the nature of their being tree-cUmbing and tree- 
dwelUng animals which hunt their prey chiefly at night. An 
oval pupil in which the long axis is vertical will cause the lens 
system to form images in which the aberrations of horizontal 
contours are greater than those belonging to vertical contours. 
The contours of trees and their branches are principally vertical, 
therefore if the illumination of the image formed on the retina 
could be increased by sacrificing the definition of horizontal 
contours it would be an advantage. This is effected by the use 
of the oval pupil since the aberration of vertical contours is little 
greater than that of a circular pupil of the same horizontal 
diameter, while the intensity of the image formed on the retina 
is as much the greater as the vertical diameter of the oval is 
greater than that of the circular pupil. 
It seems more likely to us that the function of a vertical sUt- 
shaped pupil is for protection; that is, to permit of its being 
ahnost, if not entirely, closed. In thinking of animals that have 
