508 REPORT—1890. 
requires no focussing arrangement, as it will necessarily only be used by 
approximately normally-sighted persons in military range-finding. 
The assistant observer’s instrument is of the same construction as the 
chief observer’s instrument, except that it is turned right for left, and the 
prism R is fixed into a carrier which is incapable of rotation in the frame 
F, and of course the scale is omitted. The refracting prism is in this 
case only used in the original adjustment of the instrument by the 
maker. 
The second of the three range-finders is unique in having no moving 
parts of any kind and still working from a constant base. It consists of 
two fixed-angle instruments (not necessarily setting out exactly right 
angles), similar to the assistant observer’s instrument above described, 
one right and one left hand. The instrument carried by the chief 
Fig. 12. 
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: SESSiZNAN ESSER 
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ese Nee 
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observer bears a mark M as above described ; while the assistant observer’s 
instrument is atiached to a board, say, about 15 inches long and 4 inches 
broad, wnich may be constructed of several pieces hinged together so as 
to be readily packed into a small space. Upon the board a scale of 
reciprecals is painted (fig. 14), the graduations and figures being in white 
upon a black background, and sufficiently bold to be very readily read- 
able by the chief observer from the other end of the base (20 yards of cord) 
with the aid of his telescope. The method of using the instrument is illus- 
trated in fig. 13, where A is the assistant observer’s instrument attached 
to the scale s, and B is the chief observer’s instrument. The assistant 
observer adjusts his position, as before, till he sees the mark on the chief 
observer’s instrument reflected upon the distant object. The chief 
observer then sees the scale projected by reflection upon the distant object 
at the graduation corresponding to its distance, so that the distant object 
itself forms an index or pointer for the scale. The scale can be arranged 
to rest comfortably upon the assistant observer’s shoulder when the in- 
strument is placed to his eye. The whole essential adjustment of this . 
