588 ANIMAL PHYSIOLOGY. 
parts of the retina most sensitive to both form and color; or, 
to put it otherwise, when the retina is stimulated by an object, 
whether colored or not, the mind perceives, becomes most read- . 
ily cognizant of the sensation, sees the object best, when the 
stirnulation is confined to the yellow spot; and, as will be 
learned still more fully later, all the arrangements for vision 
are directed toward the focusing of the rays of light that 
emanate from objects, so that the image may fall on this region 
of the retina. . 
_ In like manner, by looking directly forward, and having 
some one move an object in space as before, and noting when 
it ceases to be visible, an irregular figure of the field, within 
which vision ‘is distinct in varying degrees, and beyond which 
it is absolutely non-existent, may be mapped out. 
By using colored objects, as small squares of paper, by the 
above method, it may be readily learned that the field for some 
colors is much more restricted than for others; in fact, as such 
an object is moved outward, its color seems to change: thus, 
purple becomes bluish. In all retinas there is more or less 
color-blindness toward the peripheral parts, and this is espe- 
cially true of red. The field for the colors of the spectrum, etc., 
may easily be shown to be more limited than for white. 
Fie. 429.—Field of color-vision of right eye, as projected by the subject on the inner surface 
of a hemisphere. the pole of which forms the point of fixation for the eye; semi-diagram- 
matic (after Nettleship and Landolt). T, temporal side; N, nasal side ; w. boundary for 
white ; 8, for blue; R, for red; a, for green. : 
