586 ANIMAL PHYSIOLOGY. 
in the middle, but any picture appears as a photograph. It 
may be unilateral. 
2. Yellow-Blue Blindness.—The spectrum presents only red 
and green, and hence is usually much shortened. It is occa- 
sionally unilateral. 
3. Red-Green Blindness (Daltonism).—Yellow and blue may 
be discriminated, violet and blue seem alike, and red and green 
practically do not exist. 
It is to be borne in mind that it is very difficult to ascer- 
tain the exact condition of color-blind persons, from their in- 
ability to communicate their state of mind. They often make 
discriminations apparently based on color distinctions, but, in 
reality, on the form, texture, position, etc., of objects. It is 
also all but impossible to be precisely certain as to the extent 
to which the lower animals can distinguish between colors. 
To apply the above theories of color-vision to the explana- 
tion of color-blindness: In the case of red-green blindness, ac- 
cording to the Young-Helmholtz explanation, there is the ab- 
sence of one of the primary sensations (red), so that the colors 
seen are the result of mixtures of the other two primary sensa- 
tions. What we call yellow must be to the subject of this 
defect a bright green. According to Hering’s theory such 
persons lack the red-green substance; hence their color-vision 
must be limited to mixtures of yellow and blue alone. But, if 
blindness to red and green can exist separately, as has been as- 
serted, this theory fails to explain it, though the former would; 
while total color-blindness is explicable by Hering’s theory, 
but not, by the rival one. Itis probable that neither is broad 
enough to meet the facts, even if correct in principle. They 
serve the end of being provisional hypotheses till better are 
found. 
PsYCHOLOGICAL ASPECTS OF VISION. 
It is impossible to ignore entirely, in treating of the physi- 
ology of the senses, the mind, or perceiving ego. 
-By virtue of our mental constitution, we refer what we 
“see” to the external world, though it is plain that all that we 
perceive is made up of certain sensations. 
We recognize the “visual field” as that part of the outer 
world within which alone our vision can act at any one time; 
and this is, of course, smaller for one than for both eyes. 
If one takes a large sheet of paper and marks on its center 
a spot on which one or both eyes are fixed, by moving a point 
