GONIOMETER. 255 



and there clamp It fast; then revolve the tube b containing 

 the prism until the lines forming one side of the angle to be 

 measiu-ed coincide in both images, as, for instance, the lines 

 a b, a' b', fig. 166; then release the vernier and revolve it on 

 the graduated circle, until the two lines forming the other side 

 of the angle b c, V d, also coincide, fig. 1 67. The amount of 

 rotation thus obtained is the measure of the angle, or its com- 

 plement, according to the direction in which the vernier is 

 moved. Instead of starting from zero, it is of course suffi- 

 cient to take the difference of the readings in the two 

 positions. 



" There are two descriptions of angles, the one sort being 

 the plane angles produced by the lines forming the edges of 

 the planes, the other sort being the angles representing the 

 mclination of the planes themselves to each other. 



"When a plane angle is to be measured, it is necessaiy 

 that the two lines or edges forming it should be both situated 

 in a plane perfectly horizontal, that is perpendicular to the 

 axis of vision. The stage, fig. 161, furnishes every facility 

 for this adjustment, which may be known to be perfect by 

 using a suitable object glass and observing that every portion 

 of both lines remains exactly in focus on traversing the stage. 



" When the inclination of two planes is to be measured, 

 they must be so adjusted that their line of junction is parallel 

 to the axis of vision, or, to use a famihar expression, they 

 must be taken out of winding, as it is termed. A very little 

 practice will satisfy the observer that these adjustments may 

 be readily accomplished by the stage in question. 



" Similar adjustments may be eifected, although somewhat 

 more difficult, by using the forceps commonly sold with the 

 compound microscopes, more especially the three-pronged 

 forceps made by Smith. 



" The author cannot too strongly insist on the importance 

 of the microscope in examining the surface of the planes of 

 crystals subjected to measurement, convinced as he is that 

 obliquity in many cases arises not only from conchoidal frac- 

 tures, but also from imperfect laminte elevating one portion of 

 a plane, and yet allowing a very tolerable reflection when 



