hy Means of Graphical Methods. 



251 



done and recorded in the literature, need not be repeated. 

 Hence a student of crystallograplij, working as would natu- 

 rally be the case, on well-known compounds, may be able to 

 solve the majority of the problems presented for study by 

 means of graphical methods, and thus be spared much of the 

 time-consuming labor of making numerical calculations. 



Outfit. — For executing the graphical methods described in 

 this paper the following articles are needed : (1) Paper upon 

 which a graduated circle and certain stereographic scales 

 have been printed. (2) Stereographic protractors for plotting 

 and measuring. (3) Beam-compass. (1) Curved ruler. (5) 



Two triangles and a set of drawing instruments, including a 

 needle-point and hard pencils. 



The uses of most of the above-mentioned appliances have 

 already been described, and only a few suggestions concerning 

 them are now needed. The paper mentioned has a circle of 14^"^ 

 diameter, graduated to degrees, printed on it, the circle shown 

 in figure 1 being a reduced copy. Four scales, of which fre- 

 quent use will be made, are also printed on the paper. Iso. 

 1 gives the lengths of radii for describing arcs of great 

 circles ; for example, if it is known that the point D^ figure 

 1, is 32° from P^ the radius r for describing the great circle 

 ^Z^xS'may be taken directly from, the scale. Scale No. 2 gives 

 the lengths of radii for describing ares of small circles about 

 points which are located on the divided circle ; for example, 

 it being known that the point B^ figure 1, is 25° from S. and 



