Wright — Telemeter with Micrometer Screw Adjustment. 531 



Art. XLYIII. — A Telemeter with Micrometer Screw Adjust- 

 ment / by Fred. Eugene Weight. 



During the past few years many different devices have been 

 suggested for measuring the distance to a distant object by 

 merely sighting at it, and some of these, particularly the stereo- 

 comparator of Pulfrich, have proved serviceable. Three or 

 four years ago, in connection with geological field work 

 involving considerable topographic sketching, the need of such 

 an instrument was keenly felt by the writer and the following 

 apparatus devised. The apparatus is simple in construction 

 and sufficiently accurate for the purposes for which it is 

 intended. It appears, moreover, to be constructed on a prin- 

 ciple not heretofore applied to telemeters, and may, therefore, 

 be described very briefly. 



Fig. 1. 



L, 



£ 



m 



m 



The principle of its construction* is illustrated in fig. 1. 

 Light waves from a distant object strike the two telescopic 

 lenses L, and L 2 (both 50 cm focus) and after transmission are 

 reflected from the two right-angled prisms P x and P 2 to the 

 reflecting prism pair P 3 , and thence to the ocular O. The 

 incident rays are not precisely parallel and do not converge to 

 the same point in the focal plane of the ocular. They can be 

 made to do so, however, by moving the prism P 2 back parallel 

 with itself by means of the micrometer screw M until the two 

 points coincide and merge apparently into one (indicated by 

 the cross in front of the ocular). The angle between the inci- 

 dent light rays from objects at different distances is different, 

 but by moving the micrometer screw the two images resulting 

 therefrom can be brought to coincidence. 



Conversely, having once calibrated the micrometer screw 

 readings for a number of distances, it is not difficult to inter- 



* The two test telemeters which have thus far been constructed on this 

 principle were made in the workshop of the Geophysical Laboratory and 

 can be duplicated by any good maker of instruments. 



