542 COMPAEATIVB PHYSIOLOGY. 



retina being stimulated, or the original impression having 

 faded, etc. 



In the case of a top or (better seen) color-disk, painted into 

 black and white sectors, it may be observed that with a faint 

 light the different colors cease to appear distinct with a slower 

 rotation than when a bright light is used. The variation is 

 between about ^ and -jV of a second, according to the intensity 

 of the light used. Fusion is also readier with some colors than 

 others. 



It is a remarkable fact that one can distinguish as readily 

 between the quantity of light emanating from 10 and 11 candles 

 as between 100 and 110. 



The Visual Angle. — If two points be marked out with ink on 

 a sheet of white paper, so close together that they can be just 

 distinguished as two at the distance of 13 to 20 inches, then on 

 removing them a little farther away they seem to merge into 

 one. 



The principle involved may be stated thus : When the dis- . 

 tance between two points is such that they subtend a less visual 

 angle than 60 seconds, they cease to be distinguished as two. 

 Fig. 391 illustrates the visual angle. It will be noticed that a 

 larger object at a greater distance subtends the same visual 

 angle as a smaller one much nearer. The size of the retinal 



Fig. 391.— The visual angle. The object at A" appears no larger than the one at A 

 (Le Conte). 



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- 

 ever, true that when two cones are stimulated two objects are 

 inferred to exist in every case by the mind ; for the retina va- 

 ries 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- 

 tral in the nerve-cells of the part or parts of the brain con- 



