16 



parallelepiped, at the opposite ends of whose short diameter are 

 two windows on telescopic bearings to allow of the crystal being 

 seen ; the long diameter is set perpendicularly to the polarisers, 

 and the ends are heated, the temperature attained being deter- 

 mined by two thermometers inserted into holes in the box. 



The rotation of the plane of polarisation by any given thick- 

 ness of crystal is measured by the same portion of the apparatus, 

 only that the polarisers for parallel light are inserted in the 

 rings instead of those for convergent light. The ring to contain 

 the analyser has a vertical divided circle, over which a vernier on 

 the analyser works, and thus the angle through which this has 

 to turn to counteract the effect of a crystal plate between it and 

 the polariser can be read. 



The horizontal circle has further attachments for other pur- 

 poses. When it is to be used as a goniometer two telescopic 

 tubes are affixed to verticals attached to horizontal radial arms, 

 which work on pivots fixed below the circle, and are of such a 

 length as to allow each tube support to pass freely all round the 

 circle, to which each may be clamped in any particular position. 

 One of these tubes is a telescope with cross lines ; the other con- 

 tains a peculiar form of aperture, called Websky's, to admit the 

 light in a definite direction. The centre of the table is per- 

 forated, and through it passes an arrangement for fixing the 

 position and orientation of the crystal to be observed as in the 

 large Fuess's goniometer already described. 



When it is to be used to measure the index of refraction of a 

 prism, the rotating arm of the telescope is reversed in position, 

 and is clamped to the rotating arm of the table, so as to rotate 

 with it ; the light signal is made to rotate about the axis of the 

 instrument, and the adjusting arrangement in the centre is set 

 free from its clamps, and may be rotated so as to set the prism 

 in the position of minimum deviation ; contrivances for carrying 

 out all these changes are provided in the instrument. 



49. Apparatus for demonstrating double refraction by 

 pressure. 



E. 92. 1885. Made ly E. Fuess, Berlin. 



This consists of a screw vice with convex cheeks, which, by 

 means of the screw, can be made to compress a crystal placed in 

 the jaws of the vice, and this can be examined when so placed. 



50. Apparatus for producing double refraction by 

 pressure. 



E. 91. 1885. Made ly E. Fuess, Berlin. 



This is also a screw vice, but it is so arranged as to measure 

 the pressure applied. The screw end has projections to come in 

 contact with the crystal, or its holder, and forces it against a 

 movable cheek on the opposite side. This slides between bars, 

 and when no pressure is applied, stands at ; but behind it is 



