ADJUSTMENT OF FLATFISHES 475 



the dark fishes which were used did not turn pale. On the other 

 hand, the white bottom, plu^ the white loicer half, or even lower 

 fourth were found to call forth this change. These facts, which at 

 first thought would seem to indicate the operation of some other 

 factor than the relative amounts of black and white, do not in 

 themselves force us to such a conclusion. If we assume that the 

 animal's field of attention (due to the position of the eyes or other- 

 wise) extends but little above the horizontal plane, the facts 

 may, indeed, be explained on a purely quantitative basis. But 

 this is probably not the whole truth. For it seems to follow from 

 the considerations offered below that differences in the direction 

 of different portions of the visual field probably condition the reac- 

 tions of the fish in another important respect. 



This problem is more clearly allied than might at first be 

 supposed to another one which has received considerable attention 

 in the present paper. I refer to the modus operandi of the stimuli 

 which lead the fish to become very pale on a white surface and 

 ■gray upon a gray surface, irrespective of the degree of illumina- 

 tion. ^^ As I have already pointed out (probably quite needlessly), 

 a surface of pure gray reflects white fight, with no alteration except 

 a diminution of its intensity. A human observer distinguishes a 

 given object as gray, rather than white, only by reference to the 

 degree of illumination to which it is subjected, and this last he 

 infers" from the appearance of the remainder of the visual field. 

 Suppose, for example, that our visual field should for the moment 

 consist of a single uniform surface, of which we had no prior 

 knowledge, illuminated by a light of unknown intensity. Under 

 such circumstances, we should be at a loss to say whether the sur- 

 face was gray or white. ^'^ Once we have an idea of the degree of 

 illumination, however, and we make the necessary correction for 

 this, as, for example, when we view a piece of white paper in the 

 twilight, we commonly pronounce it to be white, despite the abso- 



'* The reader, if interested in this part of the discussion, is advised to refer directly 

 to the treatment of this question in the body of the text, particularly pp. 440- 

 443 and 460-467. It would be impossible, without much undesirable repetition, 

 for me to restate the entire problem here. 



^^ Colors, of course, would still be distinguished. 



