VISION IN THE MOUSE 589 



Loeb 6 suggests that some animals may localize by means of 

 changes in the form of objects, which must result from a marked 

 astigmatism existing in the eyes of certain animals. 



Cole 7 distinguishes four types of animal response to photic 

 stimuli : 



A. Response of eyeless forms. 



B. Response of forms with direction eyes. 



C. Response to size of luminous field. 



D. Response to different objects in the visual field. 



For our purpose we would make five types, dividing the last 

 into two. Of these the first is to be found in those animals which 

 perceive the presence of objects and a few general characteristics 

 concerning them. The other is in those animals which have a 

 distinct perception of form by means of a fovea or central spot of 

 most acute vision. The mouse would be included under the first 

 of these two. Under the latter would come man and the rest of 

 the primates and some other animals such as the chameleon and 

 certain birds of prey. 



Animals of the former class, destitute of a fovea, although they 

 may have a more delicate perception of the existence of objects 

 in the field of view than we, yet do not see the form of objects 

 regarded, as distinctly as we do. 



A faint star is best seen with averted gaze owing to the fact 

 that the rods are functioning and are susceptible to faint light 

 stimuli but not to distinctness of outline. This is the case with 

 the mouse, in whose retina cones do not exist. 



The visual conditions existing in the mouse as revealed by our 

 study of it are in accord with a view expressed by LeConte: 8 

 "In lower animals, especially those which are preyed upon by 

 others, it is far more important to see well in every direction than 

 to fix attention exclusively on one point, therefore, the advantages 

 of exquisite microscopic distinctness of the center of the field are 

 sacrificed for the much greater advantages of moderate distinct- 



'' Loeb, Jacques. Arch. f. d. ges. Physiol., vol. 41, p. 371. 

 7 Cole, L. J. An experimental study of the image-forming powers of various 

 types of eyes. Proc. Amer. Acad. Arts and Sciences, vol. 42, p. 410, 1907. 

 8 LeConte, J. Sight, chap. 5, New York. 1881. 



