this small volume it is not desirable, neither is it intended, to give
an exhaustive treatment on the subject of lenses and their action, but as
optics plays an important part in the transmission of photographs, both by
wireless and over ordinary conductors, the following notes relating to a
few necessary principles have been included as likely to prove of interest.

Light always travels in straight lines when in a medium of uniform density,
such as water, air, glass, etc., but on passing from one medium to another,
such as from air to water, or air to glass, the direction of the light rays
is changed, or, to use the correct term, _refracted_. This refraction of
the rays of light only takes place when the incident rays are passed
obliquely; if the incident rays are perpendicular to the surface separating
the two media they are not refracted, but continue their course in a
straight line.

All liquid and solid bodies that are sufficiently transparent to allow
light rays to pass through them possess the power of bending or refracting
the rays, the degree of refraction, as already explained, depending upon
the nature of the body.

The law relating to refraction will perhaps be better understood by means
of the following diagram. In Fig. 64 let the line AB represent the surface
of a vessel of water. The line CD, which is perpendicular to the surface of
the {127} water, is termed the _normal_, and a ray of light passed in this
direction will continue in a straight line to the point E. If, however, the
ray is passed in an oblique direction, such as ND, it will be seen that the
ray is bent or refracted in the direction DM; if the ray of light is passed
in any other oblique direction, such as JD, the refracted ray will be in
the direction DK. The angle NDC is called the _angle of incidence_ and MDE
the _angle of refraction_. If we measure accurately the line NC, we shall
find that it is 1-1/3, or more exactly 1.336, times greater than the line
EM. If we repeat this measurement with the lines JH and PK we shall find
that the line JH also bears the proportion of 1.336 to the line PK. The
line NC is called the _sine of the angle of incidence_ NDC, and EM the
_sine of the angle of refraction_ MDE.

[Illustration: FIG. 64.]

Therefore in water the sine of the angle of incidence is to the sine of the
angle of refraction as 1.336 is to 1, and this is true whatever the
position of the incident ray with respect to the surface of the w