616 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
tions in the visual purple of the rods show that they respond to the 
varying intensities of dim light, and this purple is known to disinte- 
grate most rapidly in green hght which appears brightest in twilight 
vision. Whether or not the bleached rods are active in day vision 
has not been determined. 
It is probable that all cones do not respond to color stimuli. In 
the peripheral portion of the retina there is a partially color-blind 
region where red and green can not be distinguished from one an- 
other, and the outermost portion of the retina is always totally color 
blind. Since cones occur in these areas they also must be color 
blind. From these considerations it is reasonably assumed that, in 
human vision, the ability to perceive colors depends upon the differ- 
entiation of certain of the cones. 
Since at the present time the nature of vision can not be determined 
by the microscopic examination of the retina, and since a very efli- 
cient vision may exist without color perception, it may fairly be ques-_ 
tioned whether the lower animals are capable of color vision. The 
biological importance of this problem is very great, since prevalent 
theories of the development of the colors of flowers, and the bright 
plumage of male birds, assume a color perception in insects and fe- 
male birds essentially like that in man. To learn what a bee actually 
sees has been thought impossible since it requires that one should 
possess the nervous system of an insect and still remain a man. 
There is a large literature dealing with the distinctions which 
the lower animals make between various colors, but the factor of 
intensity or brightness has seldom been satisfactorily eliminated. 
The trout fisherman is confident that one fish, at least, discrimi- 
nates colors with precision. Careful experiments with the chub, 
by feeding it from colored forceps and taking certain precautions 
to eliminate brightness, indicate that the chub distinguishes red from 
green and from blue.¢ 
Nagel, who is convinced that the phenomena of mimicry and 
warning colors demand color vision in animals, experimented with 
the dog. After taking precautions to eliminate brightness, he proved 
that the dog perceived the difference between red and blue, blue and 
green, and red and green.? 
Kinnaman tested the monkey, M/acacus rhesus. Its food was placed 
in one of six receptacles, precisely alike except that each was of a 
different color. When the monkey had learned to choose correctly 
@Washburn, M. F., and Bentley, I. M. The establishment of an association 
involving color discrimination in the creek chub. Journ. of Comp. Neur., 1996, 
vol. 16, pp. 113-125. 
+ Himstedt, F., and Nagel, W. Versuche itiber die Reizwirkung verschiede- 
ner Strahlarten auf Menschen und Tieraugen; Festschrift der Albert-Ludwigs- 
Universitit in Freiburg, 1902. 
