214 
RANGE-FINDERS, 
5000 yds. tlie length of circular arc of 1' is only 4 ft. 4*362 ins., or the 
width of two men standing in line, which would look a mere dot, even 
with good field glasses. It would appear, therefore, that an instrument 
possessing accuracy within 1' of true angle would he good enough for 
practical purposes. 
In describing the mode of using the apparatus, the writer first states 
that “No. 1 goes to the right and finds the angle. No 2 going to the 
left to find the base.” Subsequently he observes, “"No. 2 now sees 
in the angle-glass whether the reflection of the object falls to the right 
or left of No. 1. If right. No. 2 must retire; if to the left, he must 
advance, to reduce his distance from No. 1.” On reference to Fig. 4 
Fig. 4. 
No. 2. No. 1. 
it will be seen at once that if No. 2 is at D he will see some point Q 
over No. 1, and therefore object 0 will appear a distance equal to OQ 
to the right of No. 1 at B. No. 2 must therefore walk in instead of 
out. Again, if No. 2 is at C he will see some point P over No. 1, 
and therefore the object will appear a distance equal to OP to the left 
of No. 1. The proper rule for No. 2 is : when working on the left of 
a base, retire if the object is to the left and advance if it is to the 
right ; when working from right of base, the reverse. 
