which the points are to be observed, must obviously revolve in the 

 same vertical plane with the main telescope. 



For the observation of points in extreme elevations above 

 the instrument, the eyepiece prism, which may be applied interchange- 

 ably between the solar, the auxiliary and the main telescope, will 

 answer most requirements. The eyepiece prism is shown applied 

 to the solar in the cut on the proceeding page, also to the main tele- 

 scope of the 4^-in. Mining Tachymeter as shown on p. 186. For the 

 observation of points, however, in acclivities that closely approach 

 the zenith, the Duplex Diagonal Eyepiece, as shown in connection 

 with the alidade, p. 46, is probably the most serviceable attachment 

 devised for this purpose. With it, zenith observations may be 

 taken and immediately transferred to the plane of the instrument 

 by sighting through the direct ocular with which the device is also 

 provided. 



II. If very precipitous 'vertical angles are to be read between 

 points beyond the reach of the main telescope, the auxiliary tele- 

 scope should revolve upon an axis which is concentric with the 

 circle upon which such angles are to be read, and should therefore 

 be mounted in the same radial plane at one extremity of the horizon- 

 tal axis. 



These two propositions have given rise to the "top" auxiliary 

 and the "side" auxiliary telescope for collateral sighting, which have 

 been variously designed in size and method of attachment since 

 their introduction in 1855*. If, however, a mining theodolite is 

 provided with an interchangeable auxiliary telescope, all errors of 

 eccentricity may be overcome. 



The solar attachment corresponds to the top auxiliary tele- 

 scope. The adjustments prescribed in the foregoing pages cover 

 every necessity in mining work. It will be observed that the pre- 

 cautions set forth not only bring the vertical collimation plane of each 

 telescope into coincidence for all distances, but the spirit level 

 on each telescope provides a ready and accurate means for adjusting 

 the sight lines to parallelism.** 



The essentials in a mining theodolite require that stability 

 shall not be sacrificed to lightness, that the construction shall be as 

 nearly waterproof as possible, the graduations accurate and legible, 

 standards high enough to permit a complete revolution of the main 

 telescope and a partial revolution of the auxiliary, inverting tele- 

 scopes powerful up to the limit where the illumination is sensibly 

 impaired, and a horizontal axis of the most stable construction 

 capable of transferring a meridian accurately between stations close 

 together in plan and distant in elevation. 



* Sff articles by Dunbar D. Scott in Trans. .1. I. M '. E., Vols. 28. 29, eft . 



t >. ( <'. Mrtro Itr Catalog pp. 22, 52, 53 and 57. 



** Theory and Practice of Surrey inff, Johnson-Snu'th, I7f/i !:<!., l''ll,p. 

 414; also Mine Survey ing, E. B. Durham, 1913, p. 181, 



r/4 



