232 Scientific Proceedings, Royal Dublin Society. 
as a small irregular patch with short tails from it, and of some- 
what different shape according as it is viewed with the right or 
with the left eye. Now this is due to spherical aberration co-operat- 
ing with another defect which it is difficult to disentangle from 
spherical aberration, and which is caused by the light having to 
pass through the other layers of the retina before reaching the 
rods and cones. These layers, however, do little harm in the 
fovea lutea, as here they are either absent or thin, so that the 
irregular image seen when we look directly at a planet is chiefly 
due to pure spherical aberration. 
Now these defects, viz. the chromatic and spherical aberrations, 
including under the latter that further defect which arises while 
the light is crossing the retina, are dealt with in nature in the 
same way in which a photographer deals with them in his photo- 
graphic camera, viz. by limiting the aperture, which diminishes 
the effect of these imperfections. We have already found that 
the aperture of the pupil is contracted as much as is compatible 
with the other conditions to be fulfilled. Now it is evident that 
a certain amount of the defects with which we are at present 
dealing, especially when rendered less operative by the limited 
aperture of the pupil, may be allowed to remain in the eye without 
rendering it incapable of distinguishing objects separated by 1’ 
of angle, the limit already fixed by the rods and cones; and there 
can evidently be no tendency in evolution to effect any further 
improvement of the eye as an optical instrument. Accordingly, 
in persons with the best vision, the eye seems to have been just 
improved up to this point, leaving its outstanding defects still 
very conspicuous when searched for. And it is shortcoming in 
respect to these defects which is chiefly what makes one man’s 
eyesight less perfect than another’s. 
We shall next deal with another preliminary remark, which 
it is well to make, as it will dispel the oft-repeated error that 
there ought to be some connexion between our vision and the 
position of the image formed on the retina. It is pertinent to 
point this out when engaged in inquiring into the vision of 
insects, for, as we shall see presently, the image formed by com- 
pound eyes is erect, while that formed by single eyes, such as 
ours, is inverted. Neither position, however, nor a sideward 
position, nor any other, would be incompatible with our seeing 
