Wright — Telemeter with Micrometer Screw Adjustment. 533 



prism pair P 3 the image in the central part of the Held forms 

 the continuation of that in the margin. By placing the 

 reflecting prism, however, so that it is at some distance from 

 the focal plane of the ocular, the two reflected images can 

 likewise be superimposed. This was done on a trial test by 

 replacing in fig. 3 the prism P 3 by a total prism pair, thus 

 bringing the light from both L a and L 2 to a common path and 

 to reflection in a single reflecting prism at P 4 in place of the 

 prism pair there indicated. The unfavorable features of the 

 glass plate reflecting devices are chiefly the great loss of light 

 and consequent dim field, and the double image from each lens 

 which results from reflection from the two sides of each glass 

 plate. For these reasons the permanent use of glass plates in 

 this connection hardly seems feasible. Several other reflecting 

 prism devices were tried, but that of fig. 1 and its modification 

 in fig. 3 have thus far proved most satisfactory. 



The arrangement of the different parts is indicated in fig. 1 ; 

 L 1? L 2 , Pj and P 3 are rigidly fixed and stationary, while the 

 ocular O and the prism P 2 are movable, — the ocular for focus- 

 ing purposes and the prism P 2 , by means of the fine micro- 

 meter screw M 15 for the purpose of measurement. It is 

 imperative that the construction of the instrument be rigid 

 throughout. In the trial instruments thus far used, the 

 material has been either thick hard wood or a brass cylinder, 

 and of these the brass cylinder is undoubtedly the more practi- 

 cal. The three reflecting prisms P 15 P 2 and P 3 are supported 

 on their hypothenuse sides by brass blocks faced with cork, 

 and these in turn are adjustable on a brass plate. By this 

 method, the centering and adjusting of the optical parts can be 

 accomplished at any moment accurately and with little trouble. 

 The base line of the instrument to which all measurements are 

 referred is the distance L, L 2 and its length should remain 

 unchanged at all temperatures. Unless the instrument is 

 made of some non-expansible material, as invar steel, however, 

 this condition cannot be fulfilled, but for practical purposes 

 the minute changes in length which the slight temperature 

 variations produce may be neglected, since the instrument 

 itself is not one of exceeding accuracy. 



Assuming' the distance L x L 2 of fig. 1 to remain constant, 

 the theoretical accuracy of the instrument for different dis- 

 tances is not difficult to ascertain. In fig. 1, let L t L 2 = #, 



then in the triangle CDA the side CD = — . Since the dis- 

 tance of the object is always great with respect to the base of 

 the instrument L, L 2 the angle, L r object-L 2 , is small and the 

 triangle DC A may be considered without sensible error sim- 

 ilar to triangle L r Obj.-L 2 . Accordingly 



