VISION. 583 
image corresponding to 60 seconds is ‘004 mm. (4 »), and this 
is about the diameter of a single rod or cone. It is not, how- 
Fic. 425.—The visual angle. The object at A” appears no larger than the one at A (Le Conte). 
ever, true that when two cones are stimulated two objects are 
inferred to exist in every case by the mind; for the retina 
varies in different parts very greatly in general sensibility and 
in sensibility to color. 
It is noticeable that visual discriminative power can be 
greatly improved by culture, a remark which applies especially 
to colors. It seems altogether probable that the change is cen- 
tralin the nerve-cells of the part or parts of the brain con- 
cerned, especially of the cortical region, where the cell processes 
involved in vision are finally completed. 
Color-Sensations—As this subject is still in a very unsettled 
condition, it will be well in discussing it to keep the facts of 
physiology and of physics distinct from each other and from the 
theories proposed to account for them. 
It is rare to see in nature the pure colors of the spectrum ; 
more frequently the reds, blues, etc., we behold are the corre- 
sponding colors of the spectrum, with the addition of a variable 
quantity of white light. In the spectrum itself there is an 
unlimited number of shades, not usually specially noticed, in- 
termediate between the main colors. 
Hence we may regard a color as dependent on (1) the wave- 
length of its constituent rays; (2) on the quantity of the par- 
ticular light falling on the retina; and (3) on the quantity of 
white light mixed with this. When no white light at all enters, 
the color is said to be saturated, such being heavy and estheti- 
cally unattractive; when much of such light, bright, etc. A 
gray results from a certain mixture of white with black; the 
browns by fusion of red, yellow, white, and black. But in this 
and all other instances in which we speak of “fusion,” “ blend- 
ing,” “mixture,” etc., we refer to physiological blending owing 
to contemporaneous stimulation by light of different wave- 
lengths. Thus, orange results from the action of the red and 
yellow rays at the same time, and can not be produced by any 
