LXXIII
§ 182. There is a very simple instrument, called Photo
meter, for meassuring the intensity of light: In front of a white
screen we fix a black stick and place the two flames in question
at such a distance from the stick, that the two shadows produced
by them show the same darkness. The squares of the two dis
tances between the screen and the flames represents the inten
sities of the two flames.
§ 183. Shadow is caused by light falling upon an opaque
body; the partly illuminated portion of it is called Penumbra.
Applications of this are: the sun-dial and the eclipses of the
sun and moon. 362.
C. Katoptrics.
§ 184. Reflection of light. The law of reflection is this:
a ray of light falling upon a polished surface is reflected in such
a way that the angle of the incident (falling) ray with the sur
face is equal to the angle of the
reflected ray with the surface. The
straight line at right angles with
the mirror is called the normal
(C L in No. XXVI.). We always
see a body in the direction of the
rays at the moment they enter the
eye, thus not in its proper place.
The instrument for proving
this law is called Theodolite,
which consists of a telescope turn
ing round the axis of a vertical
and graduated circle.
§ 185. The law may also be
proved mathematically. On FM,
