ON SOME NEW TELEMETERS, OR RANGE-FINDERS. 501 
hitherto usually consisted of an optical square, or instrument working on 
the sextant principle, with fixed mirrors for setting out a constant angle, 
and an instrument of the same kind with mirrors capable of relative 
motion for measuring the variable angle at the base of the triangle of 
observation. Each instrument has attached to it a prominent mark—in 
the form of a vertical white line on a black background—and when in 
use the instruments are connected together by a cord of, say, 25 yards 
in length. The optical square consists essentially of a pair of mirrors 
fixed at 45° to each other (see fig. 3) ; and the variable-angle instrament 
of a pair of mirrors similarly arranged at an angle of about 45° to each 
other, one of the mirrors, however, being capable of a slight angular 
motion relatively to the other. 
In using range-finders of this class the chief observer, carrying the 
variable-angle instrument, takes up a fixed position at B (see fig. 4), 
while the assistant observer, carrying the optical square, moves round the 
FIG. 3. Fi4. 4. 
oO 
m2 
__--------C60 yds / 
chief observer as a centre, keeping the base cord taut between the 
instruments, till he finds the point a from which he sees the chief observer’s 
mark superimposed by reflection upon the distant object. When he has 
accomplished this adjustment of his position he informs the chief observer 
of the fact by shouting ‘on’; the latter meanwhile operates the mechanism 
which alters the angle set out by his instrument till he brings the mark 
on the assistant observer’s instrument into apparent coincidence with 
the distant object. The mechanism which operates the movable mirror 
also moves a scale, which indicates the distance of the object when 
correct alignment has been made by both observers. When the range 
of a moving object is being taken, the assistant observer must con- 
tinuously shift his position so as always to set out a right angle between 
the chief observer's mark and the distant object. To do this with 
sufficient accuracy is uot difficult when the enemy is approaching or 
receding in nearly a direct line to or from the observers, for the rate of 
change of direction of the base is then very slow. The difficulty is, of 
pele much greater when the enemy is moving rapidly across the field 
of view. 
