WELDON RANGE FINDER. 
635 
The mirrors M and M' are attached to the frame F, M being 
arranged to turn on the pivot at m. The adjusting-screws K and L 
limit the motion, L being set to give an inclination of 45°, and 
K of 44° 17' 1"*5. It can be used either with or without the 
telescope T, indicated in the figure, and it is evident that a telescope 
could also be applied to the prism-instrument if desired. The form 
shown in Fig. 6 could be retained and the mirror M arranged to move 
between limits fixed by adjusting-screws, as described for Fig. 9. 
No experiments were made to time the observations, but measure¬ 
ments were made very rapidly with the single instrument. Two 
observers working together would greatly hasten the operation and 
probably obtain a measurement in two or three minutes. The results 
given in the table were obtained by a single observer. 
The measuring twine and reel mentioned in Major Richardson's paper 
were not used, an ordinary tape measure being employed. 
In using either of the instruments care must be taken to keep the 
reflecting surfaces vertical, and some little variation is likely to arise 
when the observer is much above or below the object sighted on. The 
faint reflection from the surfaces of the prism is of some assistance in 
determining when the instrument is erect. 
Another form of prism is suggested by the Wollaston prism used 
in the camera-lucida, of which a section is shown in Fig, 10, A being 
Fig, 10. 
B 
90° and B 135°; light passing along the broken line ah c d ef is 
deflected 90°. For any other angle of deflection, A should be made 
A 
equal to this required angle and B should be 180°— For a deviation 
of 88° 34' 3", A should be 88° 34' 3", and B 135° 42' 58"-5. 
Thanks are due to Professors Cross and Henck and to Mr. Holman, 
of the Institute of Technology; to Mr. J. Rayner Edmands, of the 
Cambridge Harvard College Observatory ; and to Professor Cooke and 
Mr. Melville, of Harvard College, for the use of instruments and for 
valuable measurements. 
Conclusions. 
For open ground the Weldon range-finder would be very useful, and 
for general use, particularly if made adjustable to two lengths of base. 
