COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 393 



In the experiments with Acris the animal was placed beneath a 

 small glass box (approximately 4 cm. X .J cm.), large enough to allow 

 the frog to turn freely in any direction but not permitting it to hop any 

 distance. Care was taken to keep the sides of this box normal to the 

 directive axis of the apparatus, and although the intervention of the 

 glass between the animal and the light on each side introduced reflec- 

 tions which could not be avoided, it appeared, from certain tests that 

 were made, that these were sufficiently insignificant to be disregarded. 

 The method of orienting the animals at the beginning of each trial, at 

 right angles to the line joining the two lights, was similar to that em- 

 ployed in working with Ranatra. When it was desired to start with the 

 frog headed north, for example, a small incandescent light was turned 

 on at that side, while screens were placed between the animal and the 

 experimental lights. Under these conditions the fi-ogs usually turned 

 within a short time and faced the incandescent lamp. This got them 

 into the desired position, so that the incandescent lamp was then turned 

 off and the screens removed as nearly simultaneously as possible, leav- 

 ing the frog exposed to the influence of the large and small lights, one 

 at either side. The next trial was made in precisely the same way 

 except that the incandescent lamp was placed to the south instead of 

 the north, and in consequence the frog was oriented in exactly the 

 opposite direction to what it was in the previous trial. 



Four series of experiments, comprising in all 300 trials, were made 

 upon the reactions of Acris to the two lights. The first three of these, 

 (Table VIII, ^4, B, and C) were made with the lights at the same dis- 

 tance that had been used throughout all the experiments, namely, at 2 

 meters from the animal, or 4 meters apart. As has already been stated 

 (p. 345), this gave a light intensity of 1.25 to 5 CM. falling upon the 

 animal on each side. The combined results of these three series are 

 given in Table VIII. Before making series D, which is similarly sum- 

 marized in Table IX, the lights were moved nearer together until they 

 were but 2 meters apart, thus reducing the distance between them and 

 the animal by half and increasing their intensity at that, the median, 

 point four times. The intensity of light striking each side of the animal 

 was consequently then about 5 CM. to 20 CM. With the stronger 

 intensity the frogs reacted much more quickly and uniformly. They 

 sometimes acted almost immediately when the orienting light was 

 turned off and the screens removed exposing them to the experimental 

 lights ; but usually there was an interval varying from a few seconds 

 to half a minute, or even longer, before they turned toward one light 

 or the other. Parker (: 03*, p. 29) found similarly that Rana pipiens 

 responded much more quickly to the stronger light intensities. At the 



