366 
THE ROBERTS* ^RANGE-FINDER. 
2 . A light rod, about 1J in. x 1 in. in cross section, has a scale 
marked on one side. Parallel to, and slightly above, its top edge, is 
fixed a small telescope with a prism reflecting objects at a right angle. 
If desirable this rod can be made to fold into a length of 3 ft. 6 ins. 
3. A slide rule, about lOins. in length, with two scales marked on 
it. (A description of its construction is given further on.) 
When, for military topographical purposes, the range-finder is used 
in connection with a plane-table, the sights for the latter can be fixed 
on the reverse side of the calculator. 
Method of Using the Instrument. 
The tripod is planted in any convenient position. The telescope is 
focussed (its eye-piece turned so as to bring the two wires nearly 
vertical) and laid so that the centre of the left-hand wire cuts the 
object, whose range is required (Fig. I.) 
The observer then turns the prism so as to bring it opposite to the 
object glass, and, taking the graduated rod, walks to the left a distance 
of from 20 to 80 yds.; then, throwing the rod over his right shoulder, 
he looks through the rod-telescope, and so places himself as to reflect 
a fixed mark on the exterior of the tripod-telescope on to the distant 
object. When exactly in position he calls “ ready/ - ’ on which the 
assistant, who is now looking through the tripod-telescope, reads the 
number (Fig. II) opposite the left-hand wire; should the rod not 
appear in the centre of the field, he turns the sunshade till it 
does so. 
Without removing his eye from the tripod-telescope, the assistant 
next traverses the telescope so as to make the right-hand wire cut an 
index on the rod, and reads a (Fig. Ill) second number on the latter 
by the left-hand wire. The coincidence of these two readings on the 
calculator gives the range. 
The graduation of the rod is so arranged that if a base of 40 yds. 
is measured (either approximately by pacing, or accurately by placing 
the rod at such a distance from the tripod-telescope that the wires 
subtend the length on it from 16 to |), then the number which appears 
opposite to the left-hand wire (Fig. II) when the observer has placed 
himself at the right angle is the range instanter , without a second 
reading or reference to the calculator. 
When repeating a range, the observer should take a pace to his 
right or left, to ensure fresh readings on the rod. Should the second 
set of readings give the same result as before, the range found may 
be depended upon. 
In a high wind the second reading should be taken with the rod in 
a vertical position, index line uppermost, and with the eye-piece of the 
tripod-telescope turned so as to bring the wires nearly horizontal—this 
ensures accuracy. 
When, owing to fog, objects cannot be seen through telescopes, 
