The use of the two-circle contact goniometer in teaching crystallography

The two-circle contact goniometer was devised to supplement the two-circle reflecting goniometer, by furnishing a means for the measurement of medium-sized crystals not provided with sufficiently good reflecting faces for the successful use of the latter instrument. The contact goniometer has however proved itself adapted, in a greater degree even than the reflecting goniometer, to the demonstration of the principles of two-circle measurements, and their relation to geographic and astronomical measurements, and of the relations of normals to crystal faces and to projections, both stereographic and gnomonic. This simple and inexpensive instrument, a description of which is given on the following page, is thus fitted to serve a very important purpose in instructional work in crystallography.