COLE. — IMAUE-FORMING POWERS OF VARIOUS TYPES OF EYES. 405 



field, and speaks of these animals as being "attracted " by the one and 

 "repulsed" by the other. This terminology is rather misleading. It 

 should be remembered that every object upon which the light falls and 

 irom which it is retlected becomes thereby a secondary source of light 

 and must be so treated ; its size is therefore of importance as one of 

 the factors in determining the total amount of light which it reflects. 

 If the surface is what we ordinarily term black, it reflects very little 

 light, and its position may consequently represent the region of lowest 

 light intensity in the whole illuminated field. A white screen, on the 

 other hand, may reflect practically all the light that falls upon it ; and 

 if of large size, it is reasonable to predict that upon such animals as 

 Vanessa, Ranatra, and the frog it would exert a greater influence than 

 the source from which the light primarily came, provided the latter 

 were of relatively small area. Thus, if the light from a projection 

 lantern were thrown upon a large white screen and one of the above- 

 mentioned animals were placed midway between the screen and the 

 lantern, one would expect, fi-om the results of the experiments per- 

 formed on these species, to see the animal turn toward the large area 

 of light, viz., the screen. 



Loeb appears not to have considered in his experiments light re- 

 flected from surrounding objects, but in speaking of light coming from 

 more than one source, he says (Loeb :05, pp. 61, 62): "When the dif- 

 fused daylight which struck the [fly] larvae came fi'ora two windows 

 the planes of which were at an angle of 90° with each other, the paths 

 taken by the larvae lay diagonally between the two planes ; " but in 

 other places (c/! :05, p. 2, footnote) he states specifically that "if 

 there are several sources of light of unequal intensity, the light with 

 the strongest intensity determines the orientation and direction of mo- 

 tion of the animal ; " in this case apparently the animal ignores the 

 influence of the other lights altogether. 



Miss Towle (:00, p. 365), in her work on Cypridopsis, found that 

 these animals took a diagonal course due to light coming from other 

 directions beside the main source, and decided that "the resultant 

 direction [which the animal would take] could be found by compound- 

 ing all these forces if their direction and relative value were known." 

 The same question was later examined carefully, critically, and in an 

 able manner, by Holt and Lee (^01) ; but in both these papers only 

 the intensity of the various light components was considered, no ac- 

 count being taken, even in the theoretical discussion, of the possible 

 influence of the size of the areas of the various light-sources. 



In his study of Littorina, Bohn (:05, pp. 28, 29) puts the matter the 

 other way about and uses the direction taken by the animals (their 



