OPTICS. | 
Another example of the same thing is the bent appearance of a stick 
when held partly in water, the explanation being, that the light from every 
part of the stick under water is refracted, so that it seems to be raised up, 
as was the case with the coin. So, too, objects at the bottom of a clear 
stream or pond appear to be raised up, and the water seems less deep 
than it really is. The principle on which these phenomena take place 
_ is, that light when passing from a rarer to a denser medium (for example, 
water and glass are denser than air, and air is said to be rarer than water 
or glass) is refracted towards the perpendicular; and on passing from a 
denser into a rarer, is refracted from the perpendicular; and this in 
proportion to the relative velocity with which light passes through the 
different media. Thus, suppose a ray of light to pass through a piece of 
glass: on entering the glass, it is turned towards the perpendicular to a 
certain extent ; but on leaving the glass and entering the air again, it will 
be refracted from the perpendicular ; and as this must be exactly to the 
same extent as it was turned towards it‘on entering the glass, it is 
clear that the ray, on emerging from the glass, will proceed in the same 
direction as it was doing before it entered. 
Having hitherto treated of media with parallel surfaces, we will now 
consider “the case of a medium the surfaces of which are not parallel 
but are supposed to meet. A medium of this form is called a prism, 
as BAC, and the angle at which the surfaces 
meet, as A, is called the verter A ray vei 7 
of light transmitted through a prism, of 
any substance denser than the surrounding \@ n / ¢ 
medium, is always refracted from the vertex. _e 
BAC at P; let nn’ and mn’ be the perpendicu- 
lars to the two surfaces. On first entering a 
the new medium, the ray will be refracted 
from its straight course, SD, towards the per- 
pendicular, into the direction of PQ, say. Now, at first sight, it might be 
expected,that, on emerging into the air again, it would proceed in a direc- 
tion nearly the same as before entering the prism, that is, turn towards 
' the vertex of the prism; but it is clear, from the construction of 
the figure, that the ray must emerge on the opposite side, that is, turn 
away from the vertex along QR. It would be the same, although the 
incident ray were on the side of the perpendicular next to the vertex ; 
because the refracted ray in the prism must always be on the side of 
the perpendicular next the vertex, and must therefore always emerge on 
the opposite side, away from the vertex. 
_ All the effects produced on light by passing through different lenses 
Let a ray, SP, fall on the»transparent prism ™ a MG / 
Fig, 28. 
1 Latin vertex, the top or turning point, from verto, to turn, 
