GONIOMETER. 



253 



])iece, either Huyghenian or positive, sliding stiffly or screwing 

 into the tube g which fits into the tubular body of the micro- 

 scope. If the crystal be large, and no magnifying power re- 

 quired, this eye-piece may be dispensed with ; a single lens of 

 long focus may occasionally be substituted with advantage. 

 The vernier has a clamping screw at i and an adjusting screw 

 at h. A reading microscope, placed at c, is attached to the 

 vernier. Fig. 161 is the adjusting stage to support the 

 crystal whose angles are to be measured, and fig. 162 is a 



Fig. 161. 



Fig. 162. 



section of the same. The crystal may either be attached by 

 Canada balsam or wax to a glass, or blackened ivory plate / 

 dropping into the ring m, which again fits into the ring n, or 

 the crystal may be conveniently held by the clamping arms 

 in fig. 163, attached to a ring, also fitting into the plate n. 



Fig. 163. Fig. 164. 



Two of these arms have very small cups at their extremity 



