566 ANIMAL PHYSIOLOGY. 
the very few words to which the eyes are then specially di- 
rected that are seen clearly, the rest of the page appearing 
blurred; and the same holds for the objects in any small room, 
We speak of picking out an acquaintance in an audience or 
crowd, which implies that each of the individuals composing 
the throng is not distinctly seen at the same time. 2. If an ob- 
server hold up a finger before his eyes, and direct his gaze into 
the distance (relax his accommodation), presently he will be- 
hold a second shadowy finger beside the real one—i. e., he sees 
double: his eyes, being accommodated for the distant objects, 
can not adapt themselves at the same time for near ones. 3, The 
principle involved may be most precisely illustrated by Schein- 
r R 
a 
Fie. 409.—Diagram to illustrate Scheiner’s experiment (after Landois). The dotted lines indi- 
eate the circumstances under which there is double vision. 
OF 
ee 
Cc 
2 : = 
P L 
n 
Fig. 410.—To explain Scheiner’s experiment (after Bernstein). The object is at a; the lens is 
Peuresenta. by b: and the retina may be at m,m, n,7, or 1,l. The card with its holes, 
e, f, is directly in front of the lens. It is plain that, if the rays strike the retina in any wa 4 
except as represented at c, double images must be formed. One or other of these wil 
disappear according as the right or left hole of the card is stopped ; which of them will 
denend on cireumstances—i. e.. as to whether the case is that figured at m,m or 1,1. 
