Tin: MI:AM I;I.MI:.\T 



< IIYSTALS 



Irt >r axis of tin- iii>trument, the crystal to be measured may In- 

 fastened on this with wax and measured. The small crystal is then 

 mounted with the edge to lie mea>ured perpendicular to the card. 

 and the edge should coincide as nearly as possible with the axis 

 of the arm; then with the edge of the card placed on the edge 

 of the taltle and with the eye at a distance of a foot and a half, 

 the arm is revolved until one of the faces reflects the light, when 

 a reading is taken. The instrument is replaced with the light, 

 card, and eye in the same relative positions, which may be as- 

 sured by getting the reflection on the same face without moving 

 the arm ; the arm is now revolved until the second face reflects 

 the light, when another reading is taken. The difference be- 

 tween these two readings will be the angle between the poles of 

 t he t wo faces reflecting the light. Results obtained by this method 

 are more accurate than those obtained by the contact method. 

 It may be unnecessary to state that the accuracy of the measure- 

 ments is increased with the distance of the light and the eye from 

 the crystal. 



The principles of the reflecting goniometer have been sufficiently 

 illustrated by the measurement of a crystal with the card con- 



Fio. 302. The Fuess Single-Circle Reflecting Goniometer. One Quarter Natural 



Sim. 



