ZOOLOGY: S. O. MAST 
219 
6. Vision. Since the changes in the skin which result in simulation 
of the background are controlled by stimuH received through the eyes, 
the nature and accuracy of the process constitutes an excellent criterion 
of vision in so far as this term may be used in a purely objective sense. 
On the basis of this criterion it was found that, in regard to shade and 
color, vision in fishes is essentially the same as it is in man; but that in 
regard to size it is less acute. It was found that flounders distinguish 
between dots 2 mm. and 3 mm. in diameter respectively, that they 
recognize dots 1 mm. in diameter but that they do not recognize those 
0.5 mm. in diameter. 
The most interesting and convincing evidence that we -have, however, 
regarding vision, refers to motion. By means of a background consist- 
ing of a rotating disk composed of alternate black and white sectors, it 
was found that the fusion-rate of images in flounders corresponds very 
closely with that in human beings. This seems to show conclusively 
that, in regard to motion, vision in fishes is as acute as it is in man. 
There has been so much contention concerning color- vision in animals 
that it may be well to add a few words in explanation of our evidence 
bearing on it. 
On a background containing only gray or black and white no color is 
produced in flounders regardless of the shade or pattern or the intensity 
of the light. Simulation in color is consequently dependent upon the 
length of the waves of light, not upon difference in its intensity. This 
is one of the essential characteristics of color-vision in man. We have, 
however, still further evidence that bears on this matter. Flounders 
adapted to a given color tend to select a background of the same color: 
and this selection is of such a nature that it cannot be accounted for on 
the basis of difference in the intensity of the light reflected by the dif- 
ferent colors. It must, therefore, be associated with the length of the 
waves. Thus the contention that fishes have color-vision is supported 
both by the reactions of the animals and by the reactions of the chro- 
matophores in the skin. This evidence has, of course, no direct bearing 
whatever on the problem of color-sensation. 
This work is a contribution from the U. S. F. Biol. Sta., Beaufort, 
N. C, U. S. A. Presented in this form by permission of the Commis- 
sioner of Fisheries it is to be published in full, with numerous photo- 
graphs and autochromes, in the Bulletin of the U. S. Bureau of Fisheries. 
