COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 347 



III. Experiments. 



As stated in the Introduction, the experiments were undertaken 

 with the idea of ascertaining, as far as possible, to what extent com- 

 plexity in the organization of eyes is correlated with the reactions to 

 luminous areas of difterent size but of equal total luminosity. The 

 character and relative percentage of phototropic responses were used 

 as measures of the reactions. The method employed can perhaps 

 best be explained by an example. Let us suppose that an animal 

 which is decidedly positive in its ordinary reactions to directive light 

 is placed midway between two luminous areas of exactly equal shape, 

 size, and intensity, and in such a position that one light is at its right, 

 the other at its left. Let us assume, further, that each luminous area 

 is 1 cm. square, has an intensity of lUO c. p., and is situated at a dis- 

 tance of 2 meters from the animal. The measure of the light then 

 impinging upon either side of the subject would be 25 CM. (candle 

 meters). We should expect one of two results : (1) The animal being 

 equally stimulated upon both sides would go straight ahead without 

 turning ; or (2) owing to chance or random movement, it would be- 

 come turned slightly more towards one light, which would thus have a 

 more direct effect than the other, and the animal would then continue 

 crawling towards this light. But since the chance of random movements 

 in one direction is as great as in the other, the number of times that 

 the animal would go towards each of the lights should, in a large 

 number of trials, be equal, and we should have essentially a balanced 

 condition as before. 



Now let us enlarge one of the areas to, say, 100 cm. square, but keep 

 the total amount of light given by it the same as before. Its area is 

 now 10,000 times as great as before, and consequently the intensity of 

 the light radiated from a single square centimeter is now only 0.01 c. p. 

 The whole amount of light received by the animal is, however, the 

 same as before, namely, 25 CM. upon each side. If the animal is with- 

 out image-forming organs, — in other words, without eyes, — it has no 

 obvious means of appreciating the increase in size of one of the areas, 

 and we should expect the reactions to be the same as when the lights 

 were of equal size, that is, the animal would be indifferent. In this 

 case, the skin (or certain scattered cells in the skin) is the sensitive 

 surface, and since there is no apparatus for concentrating the light 

 from the large area, the amount of light received by any point on the 

 skin on either side of the animal is equal to that received by any other. 

 This is evident from the fact that light from every one of the 10,000 

 areas (each a centimeter square) which make up the large area falls 



