ON SOME NEW TELEMETERS, OR RANGE-FINDERS. 507 
Even though either of the prisms be cracked, the angles they set out will 
not be altered, though possibly vision may be impaired. Any wearing of 
the bearing of ¢ in Fr can only affect the reading by moving the index upon 
the scale by an amount equal to the slackness, but no appreciable error 
could arise from this cause by any wear that could take place after years 
of use. No doubt the instrument can be destroyed; but so long as an 
observation can be taken with it, it will be correct if the instrument was 
Fie. 11. 
originally correctly made and adjusted. In short, in a well-made instru- 
ment there can be no instrumental errors at any time which are not 
utterly negligible for military purposes, no matter how rough the treat- 
ment to which the instrument is subjected. 
The cap in which the eye-hole & is formed (fig. 12) is pivoted to the tube 
U in the manner common with telescope caps. When greater delicacy of 
observation is wanted than can be obtained by the naked eye, the cap is 
swung out of the way, and a small Galilean telescope (an opera-glass 
combination), supplied with the instrument, is inserted into the tube v. 
The telescope may have a magnifying power of, say, 24 diameters, and 
