PS 
VISION. 567 
er’s experiments (Figs. 409 and 410). Let two small holes be 
pricked in a card, at a distance from each other not greater 
than the diameter of the pupil; fix the card upright on a piece 
of board, about two feet long, and, closing one eye, observe the 
effect of looking at two pins stuck into the board in line with 
each other, at different distances apart. It may be observed that 
as soon as the nearest pin is approximated to the card within 
a certain distance it fails to be distinctly seen, and appears 
double—i. e., the near point is exceeded ; that when the distant 
pin is in focus, the near one appears double, and vice versa. 
When the image is double, blocking one of the two holes 
causes one image to disappear, and this is the right or the left 
hand image, according as the one or the other hole is stopped, 
and as it is the distant or near pin that is seen astwo. The 
reason of this will be plain from the above figures, but it must 
be remembered that an image on the right of the retina is ad- 
judged to be on the left of the visual field, as will be explained 
later. 
In what does accommodation consist ? If light from a can- 
dle or lamp be allowed to fall obliquely on the eye of a second 
person, through a card on which two triangular holes have 
been cut one above the other, three pairs of images of the flame 
(necessarily triangular) may be seen reflected from the eye of 
the observed subject, two of which are erect and one inverted; 
the brightest and most distinct being from the cornea, the sec- 
ond pair dimmer and larger from the anterior surface of the 
lens, and the smallest (c) from the posterior surface of the lens, 
inverted, since it is produced by.a concave mirror. When the 
subject of the observation 
looks at a near object, only 
one of these pairs of images 
alters appreciably, viz., that 
from the anterior surface of 
the lens, the middle pair (d). 
The conclusion then follows yg. 411 —purkinje's images. a b c during 
that accommodation consists —_egative. a, Bc, durine Positive accommo 
essentially in an alteration of 
the convexity of the anterior surface of the lens. The images 
appear nearer to each other the more convex the lens becomes. 
Without the help of a special instrument (phakoscope) the ob- 
server may fail to see the change, though that the other pairs 
do not alter position or size he may certainly readily observe. 
This change in the shape of the lens is accomplished as 
