other measures the distance between the two. The first two observers
separate, and No. 2 takes a position such that the object is reflected
to one side of observer No. 1, whom he views by direct vision. As the
object continues to move, its image gets nearer and nearer No. 1, who
during the whole of the time moves a little to one side or the other,
so as to keep the image of the object constantly in line with No. 2.
Just as the image of the object gets very near No. 1, No. 2 calls out
"Ready," the distance between the two observers is taken by the third,
and when the image of the object actually falls on No. 1 its distance
is just 25 times the distance between them, and the guns set to this
range are fired by word of command from No. 2.

[Illustration: FIG. 2. & FIG. 3.]

By using the third prism in conjunction with the second a still longer
base of one-fourth the distance of the object can be employed. The
range finder can also be used as a depleidoscope for transit
observations. For this purpose it is mounted on a block of wood by
means of elastic band and leveled by the level on its lid, being at
the same time set in the meridian of the place. The lid is opened to
make an angle with the horizon equal to the latitude of the place of
observation. On looking into the upper prism two images of the sun
will be seen on each side of the apex of the prism, which gradually
approach each other as the sun nears the meridian, and finally
coincide as it passes it, the time of which being noted gives the
longitude of the place.

Extensive trials of the instrument have been made both in this country
and in India, which agree in showing that the average error in using
the instrument is about 2½ to 3½ per cent.