COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 387 



to turn about and ^,^0 from it. Furthermore, whenever an animal was 

 subjected to both lights and turned for a number of times toward the 

 small one, it was immediately tested with the lights separately to find 

 whether it reacted negatively under these conditions. This was found 

 to be the case in enough instances to establish pretty thoroughly the 

 conclusion that a change in the character of the animal's reactions was 

 usually an explanation of a number of turnings to the small light. The 

 negative condition, however, often appears to be very temporary and 

 vacillating, and at such times the animals show much hesitancy in their 

 reactions, sometimes heading first toward one light and then toward 

 the other, as if undecided in which direction to go. When a specimen 

 appeared to have become at all permanently negative in its reactions, 

 the experiments with it were discontinued and another individual was 

 taken in its place. It will be observed in Table VII that the per cent 

 of plus reactions for the large light and for the small light were nearly 

 equal, — 87.8 per cent for the one and 83.3 per cent for the other, — 

 averaging about 85 per cent of the total number of reactions in the 

 two cases. The excess of positive over negative reactions was 76.5 per 

 cent and 68.7 per cent respectively. When both lights were used, the 

 per cent of responses to the large light was somewhat smaller, but still 

 comparatively large. This was 70.6 per cent of the total number of 

 reactions, or 45.8 per cent in excess of the reactions to the small light. 

 These figures are considerably smaller than the corresponding ones for 

 Vanessa, in which, it will be recalled, 87.2 per cent of the reactions 

 were toward the large light, while the excess in that direction was 75 

 per cent of the whole. This difference, however, is probably caused 

 not so much by an inferiority of the eyes of Ranatra, in respect to 

 image-formation, as by the variable character of its phototropic states. 

 Ranatra has periods of negative phototropism, whereas Vanessa appears 

 never to be negative in any of its locomotor responses to light. 



From the experiments which have just been described, it is apparent 

 that Ranatra is able to discriminate between a very small source of light 

 and a luminous area 41 cm. square, even when the intensity of the 

 light striking the two eyes is the same ; and judging by the large per 

 cent of reactions toward the large light by animals which are positive 

 in response to unilateral illumination, it is reasonable to infer that the 

 eyes are capable of forming images of considerable definiteness. Differ- 

 ences in the normal phototropic responses of the two animals, as well 

 as the influence of different inhibitive reactions, and other factors, will 

 not allow the drawing of very close comparisons between the relative 

 capacity of image-formation in the eyes of Vanessa and Ranatra. Other 

 things being equal, the foregoing results would indicate a more precise 



