The auxiliary telescope is the special form designed by Mr. Dunbar Scott, and it embraces all the advantages and eliminates all the disadvantages of all other types.
The particular feature is its interchangeability with top or side positions, and the means provided to ensure perfect adjustment with the minimum of trouble, thus forming a mining transit which will perform with exactness all the complex functions in mine surveying and requiring no correction for eccentricity.
The auxiliary telescope is provided with a centre that may be screwed to the threaded extension of either the transverse axis or the vertical pillars of the main telescope. In either position it is clamped firmly and ranged quickly into alignment with the main telescope by two opposing screws. The diaphragm of the auxiliary telescope has one web only, so placed that it is vertical when on the top and horizontal when at the side.
Fig. 236.—Stanley's Dunbar Scott auxiliary.
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The observation of steep horizontal angles is made only with the auxiliary on top, and of precipitous vertical angles with the auxiliary on the side. A counterpoise is provided, which exactly balances the auxiliary, so that there is no strain upon the instrument.
For vertical sighting it is also most useful and accurate, as by transferring the lines of both positions of auxiliary two lines are transferred down a shaft, at right angles to each other, which, if produced, will intersect each other exactly under the centre of the instrument, and no allowance or calculation whatever has to be made to ascertain the centre.
The whole attachment adds very little to the weight, the greater part being of aluminium, and it is packed separately in the case so as not to interfere in any way with the instrument when not in use.