12 



MINERALOGY 



construct, however delicate, to measure them. The more time 

 and patience used in both the construction of the instrument and 

 in measuring the angle, just so much nearer is the result to the 

 theoretical angle of the crystal . The instrument used for measur- 

 ing angles of crystals was invented by Carangeot in 1783. It is to 

 Rome de 1'Isle that the science owes the development of the crys- 

 tal model ; he modeled some 500 forms. It was to facilitate this 

 work that Carangeot, his assistant, devised the contact goni- 

 ometer. This form of instrument is still in use for the rough 

 measurement of large crystals and for crystals with dull faces. 

 fa Figure 11 represents the Penfield 



\ model, a very inexpensive, but 



\ convenient form, useful also as 



a protractor in laying out angles 

 and measurements in crystal 

 drawings. 



All contact goniometers are 

 constructed upon the same prin- 

 ciples, the simplicity of which is 

 such that it needs no explanation 

 other than the figure given. The 

 angle is read directly from the 

 scale. In 1809 Wollaston con- 

 structed the reflecting goniom- 

 eter, realizing the need of a more 

 accurate instrument in his work 

 on isomorphism. The principle 

 of this form is illustrated in 

 Fig. 12, where the ray of light la 

 is reflected -from the face d to the 

 eye at e, when a reading of a 

 scale attached to the instrument 

 is taken. The crystal is now re- 

 volved around the edge a, formed 

 by the intersection of the two 

 faces, the angle between which it 

 is wished to measure. -When the 

 ray of light is again reflected to 

 the eye at e, the crystal will have 



assumed the dotted position; another reading is taken and the 

 difference between the two readings will give the angle, rar', the 



V.'--'' 



