COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 339 



explained the movement of these snails landward at the time of flood 

 tide, when they are negatively phototropic, as due to the fact that, on 

 account of the bank, with bushes, grass, rocks, etc., less light came from 

 that side. Somewhat similar observations have recently been made 

 upon another species of Littorina {L. rudls) by Bohn (-OS), who found 

 that snails moving in an illuminated field took a course determined by 

 the relative amount of light coming from different directions, and that 

 if black or white vertical screens were placed in the field, the course of 

 the animals was correspondingly deflected; they were "attracted" or 

 "repulsed" by the screens, as the case might be. Bohn emphasizes 

 the importance of the size of the screen, but makes no attempt to de- 

 termine whether the greater influence of a large screen is due to the 

 formation of a correspondingly larger image upon the animal's retina 

 or merely to the fact that it reflects a greater amount of light if it is 

 white, and absorbs more if it is black. He apparently made no attempt 

 to determine exactly his light intensities, either absolute or relative, 

 and consequently his results are rather crude and only qualitative in 

 this respect. 



Torelle ("03, pp. 470, 471) made a number of tests of frogs with 

 reference to shadows and to dark objects, and found that, although 

 ordinarily positively phototropic, they move out of the sunlight into 

 the shadow, even when by so doing the movement is away fi-om, or at 

 right angles to, the direction of the ray (p. 487). When once in the 

 shadow, however, the animals turned and faced the sun-illumined area. 

 This occurred when the shadow was that of a building or merely that 

 beneath a box raised a short distance from the ground. These results 

 in their bearings upon the present investigation will be discussed more 

 fully in considering the results of my own experiments. 



The greater part of my experimental work consisted in testing the 

 reactions of various suitable animals to two sources of light, differing 

 in area, but of equal intensity at a point midway between them, where 

 the animals were exposed. It was planned so far as possible to select 

 animals with representative types of eyes, such as direction eyes (pla- 

 narians, etc.), compound or mosaic eyes (insects, crustaceans, etc.), and 

 camera eyes (vertebrates), and, in addition, to conduct parallel experi- 

 ments upon eyeless forms (such as the earthworm) which are known to 

 be sensitive to photic stimulation. For the purpose of the experiments 

 it was obviously necessary to use only animals which were decidedly 

 positive or negative in their reactions to light, since those which were 

 normally irresponsive or indifferent to light coming from one direction 

 only could not be expected to show evidence of discriminating between 

 luminous areas of different size. Furthermore, in so far as possible, 



