OPTICS. 29 
surface we might suppose the plane face of a prism. Also the other half, 
as P’AB, is equivalent-to a number of prisms all turned the other way ; 
so that the whole effect of a double-convex lens, as PP’, is to cause a 
double refraction of all rays that are transmitted through it wmwards. 
Thus, suppose,a number of parallel rays to fall on the lens PP’; let one 
fall at P ; then at the first surface it is refracted into the direction Pq, and 
if not further interfered with, it would meet the axis at q (corresponding 
to F in fig. 30), but at the second surface it is again refracted, and thrown 
more towards the thick part of the lens, so that it meets the axis at F. 
Again, of those that fall on the other side, let one fall at P’; it is refracted 
at the first surface into the direction P’g, and then at the second it is also 
turned to F. This is what takes place in the eye, when the light that 
enters by the pupil is transmitted through the crystalline lens. (HUMAN 
PHYSIOLOGY, page 74.) The point, F, at which the rays of light meet the 
axis or middle line O’q (for they all meet it at one point), is called the 
focus. In the eye this focus falls on the retina, and the cause of defective 
eyesight is simply that the rays of light are brought to a focus, not on 
the retina, but either in front of or behind it. In the former case, the 
individual is said to be short-sighted or near-sighted, and in the latter, 
far-sighted. The principle on which spectacles help to remedy those 
defects will be explained presently. After what has been said of a 
convex surface and the convex lens, it needs no proof to shew that the 
effect of a concave surface and of a double-concave lens (fig. 29) is exactly 
the reverse. As the normals to a concave surface all meet in the centre 
of the curve outside the body, inside they all diverge; and therefore 
parallel rays of ght transmitted through a medium with a concave 
surface meeting them, being all refracted towards the normals, must all 
be made to diverge also. Then as to a double-concave lens, if we 
suppose it to be the same as two prisms with their thick part outwards, 
it is at once clear that all rays transmitted through it must suffer a double 
refraction outwards, that is, be made to diverge. 
A very few words will now make the principle of spectacles perfeetly 
intelligible. In the case of a near-sighted person, the defect in his 
sight is that rays of light are brought to a focus in front of the retina, the 
cornea and crystalline lens making the rays converge too much. To 
remedy this, it is necessary to make the rays diverge a little before enter- 
ing the eye. This we saw to be done by a double-concave lens; therefore, 
near-sighted persons often wear spectacles with double-concave lenses. 
The defect in the case of far-sighted persons is that light is brought to a 
focus behind the retina: the refracting power of the cornea and crystal- 
line lens is not strong enough, and it is necessary to make the rays 
converge. This, as we saw, is the effect of a convex lens; therefore. 
far-sighted people wear spectacles with double-convex lenses. 
