15 



to fit into the lower carrier, having a large nicol between two 

 moderately convex lenses, and a centrally perforated cap to sup- 

 port the crystal, so arranged that the boundary of the perforation 

 points to 90 on the graduated rim ; (2) the analysing part con- 

 sisting of a small nicol at the ocular end, and diaphragms at both 

 ends, having minute central apertures to ensure the parallelism 

 of the light. On the upper end of this fits Calderon's stauro- 

 scope, consisting of the two halves of a rhombohedron of calcite 

 cut through the short diameter, which are then placed so as to 

 form an artificial twin, and ground into a plate, which thus con- 

 sists of two halves divided by a plane. When this plane is parallel 

 to the principal section of either nicol, the two halves have the 

 same tint, but when a crystal intervenes they will have different 

 tints, except when the plane of polarisation in the crystal coin- 

 cides with the principal plane of a nicol. The other pair, for 

 convergent light, consists of a polariser having a large nicol with 

 a single lens below, but a combination of five lenses above and a 

 crystal holder of wide aperture. The analyser has a combination 

 of four lenses at the objective end, above which is a glass micro- 

 meter, which is rendered visible at the same time as the inter- 

 ference figures by means of an ocular lens, which is surmounted 

 by a small nicol, a slit being made between the two for the 

 insertion of quartz wedge or a quarter-undulation plate. 



Secondly, there is a double stand, the uprights of which are 

 enlarged near the base into two rings, into which fit the two 

 parts for convergent polarised light just described. The uprights 

 are continued upwards as supports for a horizontal divided circle 

 with its appendages. This portion may be arranged first as an 

 axial angle measurer. When so used the analyser and polariser 

 are fixed horizontally in the tubes, leaving only so much interval 

 between them as to allow of the crystal being rotated. The crystal 

 to be examined is suspended from the centre of the horizontal circle. 

 The suspender works in an externally conical tube, which can be 

 clamped to an upright from the circle. Within this tube works 

 &, vertical shaft by which the crystal may be raised or lowered, 

 and this can be clamped to a projecting piece which works in a 

 narrow horizontal box below the circle, so as to place the crystal 

 in any position horizontally. The shaft ends downwards in a 

 Petzval's holder, or shallow curvilinear box, in which a curvi- 

 linear process connected with the crystal carrier below can work 

 tangent ially, the centre of the circular arc being about where 

 the centre of the crystal will be, which may thus be adjusted 

 to its proper orientation parallel to the faces of \,h& nicols. 

 When it is in position it is clamped to the circle, and a reading 

 taken by two verniers. The angular distance through which the 

 circle has to turn to bring one axis after the other into the centre 

 of the field of the analyser can then be read. If the angle is to be 

 measured in oil the oil carrier is placed on the small table between 

 the uprights. If it is to be measured at a high temperature the 

 enclosing box is put on the same support. This box is a long 



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