338 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ing-cloak butterfly, in the course of which he found that this insect is 

 able to discriminate between light derived from a large luminous area 

 and that from a small one, even when the light from the two sources is of 

 equal intensity as it falls on the animal, and that it usually flies toward 

 the larger areas of light. He took the biological significance of this 

 reaction to be that " this species remains in flight near the ground be- 

 cause it reacts positively to large patches of bright sunlight rather than 

 to small ones, even though the latter, as in the case of the sun, may be 

 much more intense." (Parker, -.03, p. 467.) It is evident that such 

 a reaction as that described can be taken as "a rough measure of the 

 image-forming capacity of the eyes of the butterfly studied, but a com- 

 parison of diff'erent animals on this basis can be made only in a very 

 broad and general way. 



Except for the work of Parker just mentioned, there appear to be 

 few if any direct references in the literature to experiments or observa- 

 tions calculated to determine the difference in the reactions of animals 

 to luminous (or illuminated) fields of different sizes ; at least, few if any 

 in which other factors, such as intensity and color of light, have been 

 eliminated. An experiment performed by Loeb ('90, p. 47, Versuch 2) 

 upon a species of crepuscular moth {Sphinx euphorbiae) perhaps comes 

 closest to the conditions of the present investigation. Specimens of 

 the moth were brought into a room illuminated at one side by a win- 

 dow, while upon the opposite wall was placed a kerosene lamp. Here, 

 then, were conditions with a large area of light at one side and a small 

 source of light at the other. No comparison was made of the relative 

 intensities of the lights, but as the experiment was performed at the 

 approach of twilight it is to be presumed that the light from the win- 

 dow was much less intense than it would have been in the middle of 

 the day, so that the light from the lamp was relatively more intense. 

 Animals liberated at a point midway between the window and the lamp 

 flew to the window ; and it was not until they were brought within 

 about a meter of the lamp that they flew in its direction. Loeb re- 

 gards this result as due entirely to the relative intensities of the light 

 received from the two sources ; but from the experiments to be de- 

 scribed later, it will be seen that the larger area of the window was 

 undoubtedly an important factor irrespective of, or at least in addition 

 to, the intensity of the light. 



A number of observations have been made on the tendency of ani- 

 mals to go toward, and to collect in, shaded areas. Mitsukuri observed 

 that certain Japanese marine snails (species of Littorina) gathered in 

 largest numbers in the shadow of certain objects which he used in his 

 experiments (Mitsukuri, :01, p. 1, Experiment 2), and he furthermore 



