CRYSTALL OGRAPHY MINERAL OGY. 307 



by direct vision, the two signals lying in a plane perpendicular to 

 the axis of the instrument, and at the same time to the edge to 

 be measured. The edge has to be accurately adjusted so as to be 

 coincident in direction, and as nearly as may be in position also 

 with the axis of the instrument. 



The various modifications which this instrument has undergone 

 have consisted in 



1. Such as affect the arrangements for the signals, e.g. by 



collimators. 



2. The fixing the position of the eye without having recourse to 



a second signal, e.g. by a telescope containing crossed 

 wires, or by such a telescope aided by a lens, converting it 

 into a microscope of low power. 



3. In the arrangements for fixing and adjusting the crystal in 



position. 



4. And in regard to the instrumental advantages derived from 



using methods of fine adjustment and clamping, and of 

 regulating all the parts of the instrument itself. 



The goniometer of Mitscherlich, as improved by recent modifi- 

 cations, may be said to be represented by the type of instrument, 

 whether vertical or horizontal, in which the signal is single and is 

 observed by aid of a telescope, and, when it is not sufficiently 

 distant, by the further introduction of a collimator. The crystal 

 is adjusted by two traversing motions perpendicular to each other 

 and to the axis of the instrument, effected and controlled by 

 screws, and further by two motions, also perpendicular to each 

 other, in the plane of the axis, best effected by two tangent screws 

 working on the toothed circumference of two segments of con- 

 centric circles, lying in perpendicular planes : the crystal edge 

 being approximately at the common centre of these circles. 



The latter motions serve to give to the edge to be measured 

 parallelism to the axis of the instrument j the traversing motions 

 place it in coincidence with that axis. 



X 2 



