10 



has also a fine adjustment and clamping screws in connexion. 

 This contains in its centre a vertical rod, on the top of which 

 the support for the crystal is fixed, and which can be raised or 

 lowered by means of the lowest of the small central screws 

 below the tripod table. The support for the crystal consists at 

 the base of two plates, one over the other, which can be moved 

 horizontally in two directions at right angles to each other by 

 means of screws at the sides. To the uppermost of these 

 are fixed two circular arcs one above the other, the upper one 

 having a table to which the crystal is fixed. These arcs are 

 worked by screws at the sides, so that each rotates about an 

 axis lying a little above the topmost table, and therefore some- 

 where near the edge of the crystal to be examined. The axes 

 of rotation of the two arcs are at right angles. By these 

 arrangements the crystal may, within certain limits, be placed 

 in any desired position, and moved from one position to 

 another. 



The several cylinders work in a vertical projection from the 

 tripod table, on the outside of which are two movable supports for 

 the telescopes, &c. Each of these supports consists of a central 

 ring which works on the projection, to which is fixed on one 

 side a radial arm for the telescope, and on the other a counter- 

 poise. The telescope arm may be rotated by a small projecting 

 handle, and below there is a second arm also connected with a 

 central ring, which has a tangential fine adjustment screw, and 

 a radial clamping screw. The telescopes are fixed on vertical 

 supports, and each is provided with a peculiar form of cross 

 wire, and also an analyser, to which is attached a graduated 

 circle for use with polarised light. A light signal, fixed in the 

 neighbourhood of the instrument, is reflected from one face of 

 the crystal under examination, and its image is brought to the 

 cross-lines of the telescope by the rotation of the latter. The 

 exact position of the telescope is then determined by the micro- 

 scope above. This microscope is fixed behind the objective 

 of the telescope, the tube being pierced, the objective of the 

 microscope is focussed opposite the divisions of the horizontal 

 circle, which are at a distance of 10' apart. It is illuminated 

 through an oblique cut at the side. The rays pass upwards to a 

 prism within which they are totally reflected and enter the 

 ocular of the microscope, which is provided with two parallel 

 threads. The ocular portion of the microscope can be moved 

 horizontally by means of a screw just above the telescope, so 

 as to bring the threads to coincidence with the 10' divisions. 

 The amount necessary to do this may be determined by the 

 graduation of the micrometer screw, which may be read to -gfo 

 of its circumference, and thus V of arc on the horizontal circle 

 determined. 



For a further description of this instrument see Groth, Physi- 

 kalische Krystallographie, 2nd edition, 1885, p. 567, fig, 578-9, 



