348 Proceedings of the Royal Physical Society. 



ing oca. This construction is known as the I projection, 

 and as it is more commonly employed than the last-named, 

 and as the principles of monosymmetric projection are more 

 easily grasped by considering this one first, it will be taken 

 in this place. 



66. As before, choose a radius of any length which can 

 be conveniently divided into decimal parts to three places, 

 by proportional compasses, by the decimal scale already 

 described, or by means of a good diagonal scale. Describe 

 a circle with this radius, divide it into quadrants by lines at 

 right angles to each other passing through o, the origin or 

 centre, as before. These lines are to be understood to be of 

 indefinite length. Mark each extremity of the right and 

 left axis x, and each extremity of the vertical axis z, as 

 before. Mark the bottom left-hand quadrant with the 

 Eoman numeral v., and the other three, counter-clockwise, 

 respectively vi., vii., viii., as they represent the forms on the 

 quadrants at the back of the sphere. (It may be remarked 

 here that the construction lines for stereograms in general, 

 and especially those intended for the Monosymmetric maps, 

 are better drawn first on a separate sheet of paper from that 

 intended for the finished map.) 



57. Let us take a I projection of Hornblende, as a con- 

 venient mineral to begin with. For this the axial elements 

 are — a, '55108 ; &, 1 ; c, "29376 ; and the angular elements 

 are 6b -oa, 90°; oz-oa (or P), 73° 58'. o6 (the radius) 

 being unity, the chord of /:^ is 1*2032. Take this distance 

 from the decimal scale with the dividers, and mark it on the 

 primitive in quadrants V. and vii., measuring from oz ; do 

 the same in vi. and viii. from ox. Draw diameters through 

 these points on the primitive : the line passing through v. 

 and VII. will be that on which A will be measured, and that 

 through VI. and viii. will represent c. Now set off '293 on 

 z on either side of o, and mark these points c ; also set off 

 •551 on either side of on the inclined axis in V. and vii., and 

 mark these points A. Join AC on all four sides, and produce 

 the lines both ways so as to cut the primitive, and from the 

 points where they do so describe intersecting arcs of any 

 convenient radius, and through these points draw lines 



