Deflection Prisms 



Owing to the nature of mining work observations are frequently 

 taken in the vicinity of the zenith or nadir. The ordinary transit 

 instrument can be equipped rather indifferently for this class of 

 work by applying either the ocular prism or diagonal eyepiece to 

 the eye end of the telescope, or a deflection prism to the objective. 

 The ocular prism or diagonal eyepiece require no more adjust- 

 ment than the eyepiece itself (seep. 7). After a collimated sight 

 line passes the diaphragm and the intersection of the cross wires has 

 been indelibly stamped, as it were, upon the aerial image of the 

 field, it may be manipulated or deflected in any manner that suits 

 the convenience of the operator. Like the S. F. lens, however, the 

 objective prism requires some special adjustment to harmonize w ith 

 a sight line already collimated to the optical axis. 



The Eyepiece Prism is a total reflecting prism, being one- 

 half of a cube of flint glass cut diagonally across one side. In 

 total reflection there is little loss of light from transmission or 

 absorption and none from refraction so that 

 there is accordingly produced an image of 

 great brilliancy. The field of view, how- 

 ever, is slightly reduced because the eye- 

 point is necessarily removed from the 

 position of the theoretical exit pupil. The 

 E. P. P. may be used in connection with 

 the erecting telescope but it is more appro- 

 priately associated with the inverting type, 

 113 for the single reflection surface erects the 



image in the vertical plane; the right and left sides of the field, how- 

 ever, are still reversed and this phenomenon cannot be rectified 

 unless the reflecting surface is roofed. The E. P. P. has a 

 limited range of action because the eyepoint cannot advance beyond 

 the barrier of the standards, say at about 75. It is used most 

 commonly with the mining theodolites and alidades which have 

 celestial telescopes, or with any instrument adapted for solar obser- 

 vations. In the latter case the swivel ruby glass ray filter shown 

 in the illustration is thrown into the line of sight. 



The Diagonal Eyepiece is indispensable between the limi- 

 tation just mentioned and the zenith. The optical conditions 

 are such that the power of the inverting telescope, with which it is 

 exclusively associated, is usually increased beyond its normal 

 magnification. We manufacture diagon;i s in two of the 



latest and most improved ( .1 in Fig. 114. Op- 



tically, they consist of an ocular ami an erecting system of lenses 

 interrupted by a plain total reflecting prism. The erecting system 

 gives the additional length of tube required to extend the position 

 of the eye-point and incidentally erects the image in both planes. 



