Simpler Methods in Crystallography. 405 



We now take the elements of Ehodonite and set to work 

 to plot them on the ball. We have, first, to lay down the 

 angle ft which the books informs us is the angle between 

 c and a. It is not so. It is the angle between the vertical 

 angle of reference cc^ and an axis parallel to the inclined 

 face c, as measured in the direction CA. This is quite 

 another thing. In Rhodonite the angle P is 108° 44', which 

 is measured from above on the face chosen as the front, 

 from c towards c\ As the circle of reference AC is con- 

 ventionally regarded as coinciding with ac in position, we 

 may measure upon ac^ from a and from A^c from A^ the 

 excess of /3 over a right angle, i.e., 18° 44', the chord of which 

 is -325. While the spring dividers are set to this chord, 

 describe small circles with this radius around c, c\ which 

 will be needed for the determination of the pole of the 

 crystallographic axis c, presently to be defined more 

 exactly. 



We have next to set out the angle a, which is usually 

 given as the angle between c and h. It is not so. What 

 it is really is the angle between the vertical axis of refer- 

 ence c, and a radius from o parallel to the inclination of the 

 face c as measured in the direction of CB. Had these mis- 

 leading statements not been made and copied into one book 

 after another, more of those people who have tried to learn 

 a little about crystallography might have gone farther than 

 the threshold. 



To set off a, which is 103° 18' in Ehodonite, take the 

 chord of that angle, which may be taken at 1*568, and 

 describe arcs of circles around b, b^ from c, c^ as centres. 

 Then, also from c, c^ as centres, take the chord of 13° 18', 

 which is '2316, and describe small circles cutting those 

 previously drawn and their poles of reference. The 

 points of intersection so obtained mark the poles of the 

 crystallographic axis c, which is that of the basal pinacoid 

 (001). 



The step to be taken next is to lay down the angle y, 

 which, we are told, is the angle between a and h. To set 

 this out upon the ball, take the chord of 81° 39' (the angle 

 in question in the species chosen), which is 1'307, and from 



