Wright — Telemeter with Micrometer Screw Adjustment. 535 



•005 mm and actual tests with the instruments show that changes 

 produced in the field by a movement of -01 mm of the micro- 

 meter screw can be readily detected, dx is therefore -01 mm 

 or -00001 M, and equation (4) reads 



dy—2-y' 2 -00001 

 or dy— -00002 . y 2 



At a distance of 50 meters, therefore, the probable error of 

 the instrument is *05 M, or -1 of one per cent ; at 100 M, '2 

 M of -2 of one per cent ; at 1000 M, 20 M, or 2 per cent. 



In fig. 3, a slightly different disposition of the reflecting 

 prism is shown which for a given base line is twice as accu- 

 rate as that of fig. 1, the entire base line a being used to pro- 

 duce deflections of x instead of — as in fig. 1. For this modi- 

 fication the equation reads, therefore 



a 



y = - 



x 



5 

 2 



y 



and dy = — . dx 



a 



The form of the prism pair P 4 in fig. 3 is slightly different 

 from that in fig. 1, but can be ground with equal ease. 



From the diagrams it is apparent that the images from L x 

 and L 2 do not form in precisely the same planes and are theo- 

 retically, therefore, never in perfect focus at one and the same 

 time, except for objects at an infinite distance. Experience 

 has shown, however, that if the instrument be adjusted for 

 ordinary distances, this defect is not serious, especially if a low 

 power ocular or magnifying lens of 2 to 5 cm focal length be 

 chosen. 



The inverted image produced by the instrument as shown 

 in the figures can be made upright by the use of a Rochon 

 prism <pair directly in front of the ocular. The inverted 

 image, however, is not a serious defect and equally good 

 results can be obtained without the use of the extra prism pair, 

 which encumbers the instrument and adds another adjustable 

 part to be looked after. 



Equation (3) shows that the accuracy of the instrument 

 increases with the square of its length. It seems entirely 

 feasible, therefore', to construct an instrument one or two 

 meters in length on the principle of fig. 3, with which dis- 

 tances of points within a radius of one or two kilometers or 

 miles can be read off directly with considerable accuracy, thus 

 accomplishing stadia measurements from the transit station 

 without the aid of a rodman. 



rington, 



Geophysical Laboratory, Carnegie Institution of Washi 

 Washington, D. C, June, 1908. 



