LXXVII
half a radius distant behind the mirror) towards the mirror,
its magnitude increasing from infinity to the magnitude of the
object. The image is always a virtual one.
D. Dioptrics.
§ 190. Refraction of light. When a ray of light passes
obliquely from one translucent medium into another (f.e.
from air into water), it is deflected from its straight line, and this
is called refraction. The original direction of the ray is called
the incident ray, the continuation in the new medium the
refracted ray. A perpendicular line drawn at the point of
incidence is called the normal or perpendicular and produces
an angle of incidence and an angle of refraction. Of course
a perpendicular ray is not refracted at all (it may be called the
principal ray).
§ 191. In general we may say, that the angle of the ray
of light with the normal line in a thin medium is greater than
in a dense medium, but there are exceptions to this rule. In one
and the same medium a ray of light is always refracted in the
same manner, the ratio from air to glass is 3: 2, from air to
water 4: 3, this means the angle of incidence for a ray passing
from air into glass is 1½ times as great as its angle of refraction.
(Snellius, a Dutchman discovered this law in 1625.)
§ 192. Applications are: A clear river, the bottom of which
we can see seems to be less deep than it really is. 374. We also
see fishes not in their proper place; a stick placed in water
appears broken. 375. We see the rising sun and stars a little
before they are above the horizon. 376. If we place a cube of
glass upon a written paper, the Writing seems to be raised.
§ 193. If a body passes from water into air, the angle of
refraction is greater than that of incidence and as the angle of
