28 PHYSICS. 
will now be easily understood, especially if the general principle with 
: regard to prisms be kept in 
mind, that rays of light 
transmitted through them 
are always refracted to- 
wards the thick part, be- 
cause most lenses are simply 
double prisms.. Thus, take the double-convex and the double-concave 
lenses—1 and 4 in the figure: the first is as if two prisms were fixed 
together with their vertices turned outward, and the second the 
same, only with the vertices of the prisms meeting in the middle. 
_ When a ray of light, as RI, fig. 30, falls on a convex surface, as 
' AVB, the perpendicular (or normal) at that point, NIC, is the perpen- 
dicular to the tangent-plane; and the ray being refracted towards the 
perpendicular, as IF, is there- 
fore turned towards C, the 
centre of the curve of the 
surface. Now, if a ray 
fall on any other point of 
AV, since the normal must 
always be the line from that 
Fig. 30. point to C, and the ray must 
be refracted towards the nor- 
mal, it must also be turned towards C; and so for all rays that fall 
on AV. In the same manner, all rays that fall on VB would be turned 
towards C, because they must all be refracted towards the normal at every 
point, and the normal must always point to C. The effect of the whole 
surface, AB, then, is to draw rays of light that fall on it together, » 
to a point behind the surface. Rays which draw together in this way 
are said to converge. This is the effect produced by rays of light which 
fall on the transparent cornea of the eye; they are made to converge 
and pass through the pupil; at least, by means of it, more rays pass 
through it than if there had been no refracting medium in front of the 
iris. (HUMAN PHYSIOLOGY, page 73.) 
This being the effect of one convex 
surface, it is very much greater 
when there are two together, as in 
a double-convex lens, fig. 31, which 
will be at once clear from what was 
said of the prism. The effect of the 
prism was seen to be to cause a double refraction towards the thick side ; 
now, one side of a double-conyex lens, as PAB, is equivalent to a num- 
ber of prisms all turned one way, because atievery point of the curved 

Fig. 29. 


