214 BULLETIN OF THE BUREAU OF FISHERIES. 



ground; for without this illumination the reflected light from a white background 

 always caused the fish to become maximum white. 



These experiments were later somewhat modified and repeated with an individual 

 having but one eye. Adaptation in this individual occurred very rapidly. When 

 transferred from a white background to a black, or vice versa, marked changes in shade 

 could be detected at once. In preparation for the experiment the specimen, P. afbi- 

 guttus, 27 cm. long, was put into an aquarium containing gray sand. In a short time 

 it assumed a gray shade very similar to that of the sand. It was in rather low diffused 

 daylight. By means of the apparatus described above and an additional mirror, a 

 vertical beam- of direct sunlight 3 mm. in diameter was reflected down into the eye. 

 The beam of light was so small that none of the skin around the eye, except the projected 

 margin over it, became illuminated. The illumination was continued for 30 minutes. 

 During this time the fish did not move and there was but little movement in the eye, 

 which was merely drawn down into the socket occasionally, as it ordinarily is when 

 stimulated by contact. Thus the illumination from above as compared with the reflected 

 light from the background was very much stronger than it normally is and, in accord 

 with Keeble and Gamble's ratio theory, the fish should have become darker, but no 

 change in shade was detected. 



The same was, however, also true when the experiment was repeated beginning 

 with the animal adapted to a black background. In this experiment the fish was put 

 into a black aquarium, and after it became adapted it was suddenly and entirely covered 

 with gray sand. The apparatus was previously adjusted, so that as soon as the sand 

 was removed from the eye the beam of light entered it. The posterior portion of the 

 animal was uncovered for observation as in the preceding experiment. The illumination 

 was continued from 11.35 a. m. to 12 m. No appreciable change occurred in the skin. 

 If anything it became somewhat lighter. 



At 12 m. the beam of light was intercepted without making any other change. At 

 12.10 p. m. the fish had clearly become somewhat lighter, and at i p. m. much lighter, 

 showing that the absence of changes in the skin under the apparatus was in all probability 

 not due to lack of time. 



These experiments were again repeated with the same individual, but with the 

 beam of light entering the eye at an angle of 35 degrees with the horizontal. There 

 was, however, no difference in the results obtained. Moreover, the fish was placed in 

 direct sunlight and a small shadow cast on the eye so as to reduce the light received 

 from above without decreasing that received from the background. Under these 

 conditions, in accord with the Keeble and Gamble's ratio hypothesis, the fish should have 

 become lighter, but, although the shadow was held continuously on the eye for 40 

 minutes, no appreciable change in the shade of the skin occurred. While these results 

 are not conclusive, they do throw considerable doubt on the validity of this hypothesis. 

 It is, however, certain, as previously demonstrated, that simulation of the background 

 is not controlled solely by light reflected from the bottom; i. e., the effect of the light 

 received from the background must be modified, in some way, by light received from 

 some other source. This modification is, however, in all probability, not so simple as 

 is demanded by the hypothesis in question. 



