VISION. 589 
Influence of the Pigment of the Macula Lutea.—If we inter- 
pose a solution of chrome alum between the eye and a white 
cloud while the general field is purplish, a rosy patch appears 
in a position corresponding to the yellow spot. This is owing 
to the fact that the solution allows only the red and greenish- 
blue rays to pass, and, the latter being absorbed by the yellow 
spot, we see only the former in the part of the field of vision 
corresponding to this area. The experiment is also an excellent 
one to mark out the site of the spot. Since the macula lutea 
is the part of the retina concerned in the usual so-called “ di- 
rect” vision, it will be evident that what would be yellow but 
for the influence of the pigment of this spot appears to us 
white. 
After-Images, ete.—Positive after-images have already been 
referred to; but an entirely different result, owing to exhaus- 
tion of the retina, may follow when the eye is turned from the 
object. If, after gazing some seconds at the sun, one turns away 
or merely closes the eyes, he may see black suns. In like man- 
ner, when one turns to a gray surface after keeping the eyes 
fixed on a black spot on a white ground, he will see a light spot. 
Such are termed negative after-images, and these may them- 
selves be colored, as when one turns from a red to a white sur- 
face and sees the latter green. They may be explained upon 
either theory of color-vision. According to the theory of 
Young and Helmholtz, in the latter case the green appears be- 
cause the primary color-sensation for red is exhausted, while 
the others become more prominent accordingly ; but it is more 
difficult to explain the black suns, etc., by this theory, though 
it is, of course, open to suppose that all the primary color-sen- 
sations have been exhausted. 
According to Hering’s theory, the dark after-images as well 
as the colored ones are the result of the preponderance of one 
or the other of the two processes of assimilation and dissimila- 
tion, But, in truth, the subject is very difficult of complete 
solution at all by the kind of explanations we are at present 
employing. 
It is of some importance to remember that the retina is not 
equally sensitive to all colors. We see the blues of evening 
more readily than the reds or yellows, hence the employment 
of the former extensively by artists in depicting evening 
scenes, 
Since there is a maximum point of stimulation for each main 
color, it is possible to understand how, by increase of the inten- 
