the angle included between the reflecting surfaces is 45. Slight 

 movements, therefore, in the plane of reflection will produce no 

 error in the path of the reflected beam. 



Fig. iig 



Fig. 120 shows how the penta prism is mounted in a separate 

 housing with a clamp for securing it to the collar of the objective. 

 It is also provided with an axis 

 of rotation, normal to the tele- 

 scope, that is provided with 

 a separate small clamp-and- 

 tangent movement. The most 

 reliable results are now pos- 

 sible because of the auto-cor- 

 rection in the prism itself and 

 the constant amount of ec- 

 centricity due to the fixed 

 position of the objectives sup- 

 plied with our Tachymeters 

 and Theodolites. The attach- 

 ment applied in this way to 

 any ordinary instrument converts it suitably for mining surveys. 

 Vertical angles only would have to be corrected for eccentric! t\ nt 

 collateral sighting. 



Adjustment 



To use the appliance for the observation of a very precipitous 

 horizontal angle, sight some point with the verticle wire while the 

 telescope is tipped downward, all clamps set. Apply the penta 

 prism to the objective collar and clamp securely. Open the prism- 

 box axis, elevate the telescope (adding if necessary the eyepiece 

 prism) and rotate the objective prism, first by hand then with its own 





