ON SOME NEW TELEMETERS, OR RANGE-FINDERS. 503 
over the right-angled isosceles prisms (fig. 6) used in the Weldon range- 
finder, in that the field of view is very much larger.! 
The difficulties of constructing such prisms so accurately that no error 
is perceptible is so great that we have had recourse to a very simple, 
expeditious, and cheap method of correcting them without regrinding. 
This consists in providing, in the path of one of the beams of light (either 
that passing through the prism from the mark upon the other instrument, 
or that of direct vision over or under the prism), a refracting prism of 
very small angle, rotating it in its own plane till the adjustment is perfect, 
and then fixing it securely in this position. This refracting prism may 
be cemented on to the face Bc of the prism (fig. 5), but we prefer to fix 
it separately in front of the prism in the direct beam, as shown in fig. 7, 
where the angle of the prism is greatly exaggerated. It will be seen that 
when the prism is in the position shown by the full lines the angle set 
out will be the angle between the lines mr and H 0’; whereas, when the 
prism is rotated through 180° in its own plane so as to be in the position 
FI Fa. 6. 
ae ee 
or 
indicated by the dotted lines, the angle set out will be that between the 
lines MF and uo”. For intermediate positions of the refracting prism 
the angle set out will lie between those extremes. In this way the angle 
can be made 90° to any desired degree of accuracy. It must be under- 
stood that the angle of deviation of this refracting prism need never 
exceed a few minutes. 
The variable-angle instrument, carried by the chief observer, is con- 
structed in exactly the same manner, except that the refracting prism is 
of larger angle, and instead of fixing this prism in one position it is left 
free to be rotated relatively to the frame carrying the reflecting prism. 
The disc in which the refracting prism is held is provided with a scale 
marked upon its circumference, and an index is provided upon the frame 
in which the reflecting prism is fixed. The scale is marked to give the 
distance in yards of the object whose range is required. 
The nature of the scale is indicated in fig. 8. It will be evident that a 
ray of light GH (supposed reversed in direction so as to proceed outwards 
from the eye to H) (fig. 7) will, after passing through the refracting prism, 
1 Such pentagonal prisms, we now find, have already been used for some time 
on the Continent. 
