THE ELEMENTS OF MICROSCOPY.—THE HUMAN EYE. 299 
is, the rays from its periphery are brought to a focus in a different 
plane to those occupying a central position. 
Now, this fault may be illustrated by a diagram, which may now 
be thrown upon the screen. (Fig. 28.) 

But although it is so easily shown in a diagram, I am not so sure 
that a small amount of spherical aberration is easily detected by 
the student. It appears as a haze or fog of light over the object. 
I have here a lens strongly afflicted with spherical aberration, and 
will show you at the close of the paper several objects with it. 
In the human eye this defect is not observable to any great 
degree, as the peripheral or more strongly refracting rays are cut 
off by the iris. Then, again, the curvature of the cornea is ellip- 
soidal rather than circular, so that the rays farthest from the axis 
are least deviated, while the two curves of the crystalline lens 
correct, so to speak, the one the other; and, lastly, this lens is of 
such construction that its refractive power diminishes from the 
centre to the circumference. 
Another defect in the eye is due to the different meridians having 
dissimilar degrees of curvature. 
If these concentric circles are observed 
with one eye, they are seldom all dis- 
tinct at the same time, and there is 
produced a kind of Maltese-cross effect, 
not perceivable, perhaps, in many in- 
stances with large circles, but are notice- 
able when drawn to such a size that the 
outer one is about two inches in dia- 
meter. (Fig. 29.) 
This defect is called astigmatism, 
and known to oculists as a common 
cause of headaches. Spasm of the 
focussing apparatus may derange the sphericity of the eye, and 
so affect vision. Strained vision is very liable to this. On the 
other hand, the same apparatus may be paralysed, and ordinary 
vision deficient, whilst the focussing of the microscope might 
possibly correct it. 
Astigmatism has injuriously affected painters ; Turner for instance, 
whose later pictures are discovered to be slightly distorted, in 

Fig. 29. 


