DESCRIPTION 
OF A 
RANGE-FINDER 
PROPOSED BY 
CAPTAIN F. ROBERTS, R.A., 
BY 
LIEUTENANT H. C. DUNLOP, R.A. 
In 1875, the Okehampton experimental committee recommended 
that “ every battery in the field should be provided with a powerful 
telescope, mounted on a tripod.” It was to add to the efficiency of 
such a telescope, that the range-finder, of which the following is a 
description, was devised :— 
The instrument requires one observer, with an assistant, to work it, 
and consists of,— 
(1) A telescope on a tripod. 
(2) A graduated rod about 6 ft. long. 
(3) A simple calculator. 
1. In the focus of the telescope are placed two parallel lines (either 
cut on glass or made of the ordinary spiders web) which subtend a 
fixed angle (tan ~). The sunshade of the telescope carries a double 
reflecting prism (angle reflected=90°), which can be moved at pleasure 
opposite to, or away from, the object glass. 
The tripod is fitted with a head which gives elevating and traversing 
motion. 
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