532 Wright — Telemeter with Micrometer Screw Adjustment. 



polate and to draw a curve indicating the distance away of 

 any object in terms of micrometer screw readings. The 

 equation expressing the relation between micrometer screw 

 readings and distances away of objects is derived below 

 (page 534). 



The optical parts of the instrument are the following : 

 Two achromatic plano-convex telescopic lenses, 50 cm focal 

 length and 25 mm diameter; two right angle reflecting prisms 

 P, and P 2 , 18 mm length of side ; a reflecting prism pair P 3 con- 

 sisting of two right-angled reflecting prisms, the larger one 

 of 20 mm edge and the smaller one of 14 mm edge, its hypothenuse 



Fig. 2a. 



Fig. 26. 



N/ 



face fitting the side of the larger prism closely. The hypothe- 

 nuse surface of this smaller prism is silvered ; at its center, a 

 round circular portion 3 to 5 mm in diameter has been removed 

 as in the Abbe camera lucida reflecting prisms. The two 

 prisms are cemented with Canada balsam, the light from the 

 larger prism reaching the ocular through the central part of 

 the field, while that from the smaller prism is reflected by the 

 peripheral silvered margin. 



Experiments with this type of double reflecting prism have 

 indicated that the instrument should be so built that for aver- 

 age distances the round aperture in the center of the field 

 should be situated near the focal plane of the ocular O. 



Other reflecting devices were tried in place of the prism 

 pair P 3 , two of which are shown in figs. 2a and 2h, in which 

 plane parallel glass plates are used. The four glass plates of 

 fig. 2a are very difficult to adjust accurately and for the pur- 

 pose special adjustment facilities had to be constructed. The 

 device of fig. 2b consists of two glass plates and the light rays 

 follow the paths indicated by the arrowed lines. With the glass 

 plate devices the images are superimposed, while with the 



