356 
THE WELDON RANGE-FINDER. 
prism be taken it will be found to act very much as Major Weldon's 
prism, winch has equal base angles as Edwards' infers, but has not an 
apex angle of about 90J. Furthermore, an instrument maker has been 
found who offers to grind angles to an accuracy of 1", and who offered 
to do so for 25 francs instead of £30. 
Edwards' is quite right about my description of the duplex mirror 
arrangement. I made a careless, but very obvious error in saying the 
mirrors are inclined to one another at 88° 34' 3". A glance at the 
instrument would prove this, and I fancy no one could be misled by 
such an obvious inaccuracy. 
Edwards', again, is quite wrong in his last para., and a glance at his 
figure proves it. If the reflection of the object falls to the right of 
No. 1, No. 2 would have to retire. I have not yet found a man I have 
been instructing who did not at once, on seeing the reflection of the 
object, 0, in the position O', state that he saw the object to the “ right 
of No. 1." He has then to retire. I did not say on No. l's right— 
and I preferred to use the simpler expression, as it is one in constant 
use of the men under instruction. 
I remain, 
Yours very truly, 
JOHN B. RICHARDSON. 
