254 USE or the microscope. 



fitted with a piece of India rubber or cork, by means of which 

 they can be pressed against a crystal without bruising it. The 

 third arm has a small fork at its extremity, which tiu-ns round 

 on the end of the arm at x in any direction to suit the crystal. 

 The arrangement of screws and guides shown in the drawing 

 is for the purpose of sliding the arms to and from the centre 

 where the crystal is to be fixed. Fig. 164 is another ring 

 fitting into n with two light springs to hold a slip of glass on 

 which to fix a crystal when desired, or to carry one of the 

 ordinarily mounted microscopic specimens. 



" p, figs. 161 and 162, is a semicircle attached to the ring 

 n, which is centred on two screws, at t f, passing through 

 two uprights, so that it may be inclined at any angle and 

 fixed by the clamping screw r. This semicircle may be 

 graduated, in which case it can also be employed for deter- 

 mining the inclination of the optic axes by polarized light. 

 Not only can the plate m be revolved in any direction within 

 the ring n, and set to any inclination by the semicircle p, but 

 the whole ring o carrying the upright supports may also be 

 revolved around the short tube shown in the section at ?t 

 upon the plate s, which fits upon the traversing stage of the 

 microscope. The ring o may also be graduated, if thought 

 useful, and thus rendered convenient for investigations by 

 means of polarized light. 



" When a crystal, or any angle of the same, is viewed through 

 the prism attached to the goniometer, two images thereof are 

 produced by revolving the prism, which, with respect to each 

 other, may be made to occupy various relative positions, as 

 shown, for example, in figs. 165, 166, 167. Let a b c, fig. 



y 



Fig. 165. Pig. 166. Pig. 167. 



165, be the angle to be measured, the crystal being adjusted 

 properly, as hereinafter explained. Place the vernier at zero. 



