OF THE GONIOMETER. 



The instruments used for measuring the angles at 

 which the planes of crystals meet, or, as it is frequently 

 expressed, incline to each other, are called goniometers. 



Let us suppose the angle required at which the 

 planes a, and Z>, of fig. 50, incline to each other. 



The inclination of those planes is determined by the 

 portion of a circle which would be intercepted by two 

 lines ed, e f, drawn upon them from any point e of the 

 edge formed by their meeting, and perpendicular to 

 that edge the point e being supposed to stand in the 

 centre of the circle. 



Fig. 51. 



Now it is known that if two right lines as gf, d h, 

 fig. 51, cross each other in any direction, the opposite 

 angles def, geh, are equal. 



