WELDON RANGE FINDER, 
633 
it, estimates that- a number could be made correctly for about $2 apiece, 
as several could be ground together. 
It is altogether probable that the true Weldon prism is similar to 
prism No. 2, and that prism No. 1 was made from the imperfect 
description given in Major Richardson's paper, where no allusion is 
made to the size of angles B and 0. 
Fig. 8 shows a modification of the frame of the prism found con¬ 
venient in practice. A metal screen, BDD' B', 
perpendicular to the back, projects at the side 
next the angle B, and this serves to cut off 
annoying reflections, while it prevents the use of 
the wrong angle in observing. To obtain the 
reflection of an object on the observer's right 
he looks into the face A C C' A' in a direction 
nearly parallel to C B, and for an object on his 
left he looks into the face ABB'A' in a direction 
nearly parallel to D B. If the prism is ever 
moved from its frame, care must be taken to 
replace the smallest angle next the screen. 
A serious difficulty arises in using the instrument when the ground 
is not very open, as it is then often impossible to see, from both ends 
of the required base, the object whose distance is desired. This 
difficulty would be likely to arise to some degree with most range¬ 
finders dependent on angular measurements, unless, as in the Berdan 
telemeter, the base were very short. The trouble could be considerably 
reduced by using in connection with the Weldon range-finder a prism 
made to reflect an angle of 90°, such as that described above. One 
observer having fixed the point C (Fig. 3) with the Weldon instrument. 
Fig. 3. 
another observer with the right-angled prism moves along the line 
CDF until he sees A reflected from the direction C. He will then be 
at D, midway between C and B, and in this way an obstacle to the 
view from B might be avoided. The new base being half the usual 
one, the multiplier would have to be twice that required for the Weldon 
instrument alone, and the error of observation would be somewhat 
increased. The right-angled prism and the Weldon prism might be 
fixed back to back on the same frame, so that each observer should have 
both at command. No screen would be needed for the right-angled prism. 
Fig. 8. 
c_B 
