in the Study of Crystallography. 163 



a face is specified by giving the angle which the plane of 

 the great circle which represents it makes with the plane 

 of the equator, or great circle No. 0, and the azimuth of one 

 of the nodes of intersection of the two circles. An edge is 

 specified by azimuth and altitude of the node marking its 

 extremity in the positive hemisphere. As it is a diameter of 

 the sphere its direction is fixed. 



As the measurements with the metrosphere used may vary 

 to the extent of half a degree, angles and arcs whether 

 observed or calculated are stated in degrees, and half or 

 quarter degrees. It is impossible to illustrate examples with- 

 out the aid of the globe itself, and the reader should have 

 one before him in order to make for himself the construction 

 described. The black or white globe with metrosphere 

 mentioned in the paper is, on the whole, the best that is to be 

 had in the market. The sphere Lejeune, to be had of the 

 same publishers, is much more elaborate and admits of arcs 

 being laid down and measured with greater accuracy and 

 precision than the metrosphere of de Magnac, but it is less 

 handy and costs four times the money. The handiness of the 

 metrosphere is its great recommendation. One metrosphere 

 can be used with any number of globes ; so that separate 

 details can be worked out on separate globes and combined 

 on others, thus avoiding the risk of mistakes due to over- 

 crowding. As extra globes for the metrosphere cost only 

 ten francs each, the expense is small compared with the 

 convenience afforded. 



In the absence of a globe especially constructed for drawing, 

 an ordinary terrestrial or celestial globe may be used w T ith 

 profit. It should be fully mounted, with complete brass 

 meridian and wooden, or metal, equator, and should be 

 furnished with a flexible brass quadrant to be attached to the 

 meridian in the pole of the observer or elsewhere. If now 

 the brass meridian be brought into such a position that the 

 axis of rotation of the globe, which already lies in the plane 

 of the meridian, comes to lie also in that of the equator, and 

 the quadrant be attached to the meridian in the pole of the 

 equator, we have an exact representation of the metrosphere, 

 excepting that the circles in the ordinary mounting stand a 

 little way off the globe, while those of the metrosphere touch 

 and rest on it. The principal great circles such as the 

 equator, ecliptic, and prime meridian are always clearly 

 marked on a globe, and they are valuable as circles of 

 reference. One of them is taken as circle No. 0. Great 

 circles can be easily shown on the ordinary globe by pieces 

 of twine or thin tape joined up at their extremities by an 



