620 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
tinged with red or green. Thus there are variations in trichromatic 
vision. Greater abnormalities may take the form of dichromatic 
and monochromatic vision. The latter is a rare pathological condi- 
tion in which all colors are perceived as shades of one; vision, there- 
fore, is essentially colorless (achromatic), the images obtained being 
comparable with photographs. In dichromatic vision color percep- 
tion is so limited that all of the shades perceived may be made by 
combining two of the spectral colors red, green, and blue; blind- 
ness to the third of these colors may be partial or complete. The 
ordinary color blindness is dichromatic. Forty men and four women 
per thousand are either wholly unable to perceive certain colors or 
can recognize them only with difficulty. This defect is usually con- 
genital and hereditary. It may cause so little trouble as to pass unde- 
tected until the age of seventy. All attempts to overcome the color 
blindness by educating the color sense in various ways have failed. 
Since dichromatic color blindness plays so large a part in the 
theories of normal vision, a portion of Doctor Pole’s description of 
his own case is here inserted. He says,¢ “ In the first place we see 
white and black and their intermediate gray, provided they are free 
from alloy with other colors, precisely as others do. (Such state- 
ments are confirmed by those who are color blind in one eye, the other 
being normal.) Secondly, there are two colors, namely, yellow and 
blue, which also if unalloyed we see, so far as can be ascertained, in 
the normal manner. But these two are the only colors of which we 
have any sensation. It may naturally be asked: Do we not see objects 
of other colors, such as roses, grass, violets, oranges, and so on? The 
answer is that we do see all these things, but that they do not give us 
the color sensation correctly belonging to them; their colors appear 
to us as varieties of the other color sensations which we are able to 
receive. Take, for example, the color red. <A soldier’s coat or a stick 
of sealing wax conveys to me a very positive sensation of color, by 
which I am perfectly able to identify, in a great number of instances, 
bodies of this hue. But when I examine more closely what I really 
see, I am obliged to conclude that it is simply a modification of one 
of my other sensations, namely, yellow. It is in fact a yellow shaded 
with black or gray, a darkened yellow or yellow brown.” 
Dichromatic vision occurs in three forms, in two or which red and 
green are not differentiated from one another. The three forms are 
named protanopia, deuteranopia, and tritanopia, respectively. In 
protanopia the red end of the spectrum is shortened; that is, a portion 
which to the normal person is red appears black. The remainder of 
the red, the orange, the yellow, and the green appear as successively 
“Pole, W. Color blindness in relation to the Homeric expressions for color. 
Nature, 1878, vol. 18, pp. 676-679. 
