tangent movement, until the test point is again cut by the vertical 

 wire. The reflecting surfaces now lie normal to the vertical plane 

 of the instrument. 



Observing these simple precautions the objective penta prism 

 is now ready to perform all the functions of a top auxiliary tele- 

 scope. It has the advantage of adding only three ounces to the 

 weight and of utilizing the power of the main telescope for aux- 

 iliary sightingat the cost only of the light which is absorbed in the 

 glass. It is not guaranteed, 

 nor is it necessary, for the 

 observation of horizontal 

 angles that thedeflectedsight 

 shall be truly at right angles 

 to the collimation line. 

 Optical plumbing is rarely 

 attempted in America, but 

 it may be accomplished 

 with this appliance by Fig. 121 



sighting in points on the four quadrants and by locating an in- 

 tersection between them. Fig. 121 shows the path of the six princi- 

 pal rays passing through the correction wedge, the penta prism and 

 the secondary objective as supplied with our Prismatic Mining 

 Tachymeter. This instrument was designed to utilize the principle 

 of interchangeability between the principal objective and a second- 

 ary objective mounted in the outer extension of a perforated 

 horizontal axis. 



For the observation of vertical angles with the prism mounted 

 as in Fig. 120, a correction for eccentricity would have to be ap- 

 plied (see p. 175), and for this purpose accurate right-angle deflec- 

 tion would have to be ac- 

 complished by the correc- 

 tion wedge as above sug- 

 gested. To overcome this 

 necessity we opened up the 

 horizontal axis and by 

 utili/ing the secondary 

 objective referred to above 

 were enabled to increase the 

 power of collateral sighting 

 and use the penta prism 

 with the same facility as an 



Fi f . 122 interchangeable auxiliary 



telescope. The system possesses further advantage in that all 

 observations may be conducted directly from the ocular of the main 

 telescope by utili/.ing a small inteiiur penta prism which can be 

 thrown into, or out of, the principal sight line as desired. 



196 



