VISION. 573 
does not increase regularly from the center to the circumfer- 
ence. 
Astigmatism.—In this defect the vertical meridian is sup- 
posed to be more convex than the horizontal, as is partially 
the case with the cornea of the eye, and it is to this body that 
astigmatism is usually referable, rather than to the lens, though 
the latter may also be defective. 
In astigmatism, when a vertical line is in focus a horizontal 
can not be distinctly seen, and the reverse. This any one 
may readily demonstrate to himself by drawing one straight 
line at right angles to the center of another and looking at the 
figure; when the one is seen distinctly, the other is blurred. It 
is to be borne in mind that, in order to see a horizontal line 
distinctly, it is of most importance that the rays that diverge 
from this line, in a series of vertical planes, be well focused, 
rather than those which diverge in the plane of the line itself; 
.so that, when the cornea is most curved in the vertical meridian, 
a horizontal line will be represented by an image of a horizontal 
line at the nearer focus—i. e., when the vertical is the most con- 
vex meridian, horizontal lines are soonest focused, and this 
holds, in fact, of most eyes. 
When the astigmatism affects several meridians, “irregular 
astigmatism ” results. , 
The defect in question is to be corrected by glasses made of 
sections of a cylinder, thickest in the region corresponding to 
that of greatest corneal, etc., defect. 
Chromatic Aberration.—In the figure below, in which hh rep- 
resents the lens, it will be seen that the violet and red rays 
have different foci, so that, when the eye is accommodated for 
the one set of rays, the others are seen indistinctly. Assuming 
Fie. 415.—Diagram to illustrate chromatic aberration (after Foster). 
that the retina is at f, the rays will be blended; but if between 
Vand f, or f and R, the blue center will have a red circumfer- 
ence, and the reverse respectively. 
As the focal distances for near objects differs so little usual- 
ly, this defect is not observed by us; but it may be made ob- 
