Simpler Methods in Crystallogra'phy. 327 



large collection of Scottish Minerals in the Edinburgh 

 Museum of Science and Art, I have set out the poles of 

 several species of minerals on four-inch wooden balls by 

 means of a more complex instrument for laying down 

 spherical triangles than the simple device here described. 

 The Goniograph referred to, which was made by the Museum 

 staff to my design, is exhibited amongst the scientific instru- 

 ments in the Edinburgh Museum of Science and Art. 



12. But it must be obvious that where much crystallo- 

 graphic work of a varied nature has to be done, the wooden 

 balls, useful as they are, sometimes prove a little incon- 

 venient to carry about, just as a geographer or a seafaring 

 man would find it inconvenient to carry about a terrestrial 

 globe of any useful size. We are thus led to seek for some 

 suitable chart or map which shall represent in one plane all 

 the forms represented on the solid. 



13. Here a difdculty is encountered, because it is not 

 possible by any means to transfer the poles and zones from 

 a spherical surface to a plane one, without a certain amount 

 of distortion so far as area is concerned. But it is well to 

 remember in this connection that in Crystallography we are 

 concerned only with directions and not with magnitudes, 

 so that this apparent defect is really of no consequence. 



14. There are several methods of representing crystallo- 

 graphic zones and poles on a plane surface — each method 

 possessing some defect, and at the same time presenting 

 advantages of its own. An excellent method, much used in 

 former times on the Continent, is Quenstedt's Linear Pro- 

 jection, of which a good account is given in several books on 

 mineralogy, notably in Groth's Physicalische Crystallographies 

 and in Bauermann's Mineralogy. It is also described in 

 Dana's Manual. As these are easily seen, and as the method 

 of projection offers no special difficulty, it need not be further 

 noticed here, especially as it has no very evident connection 

 with the subject of this paper. 



15. Another method of projection which is very useful in 

 certain cases, is that which is usually called the Gnomonic 

 Projection. For the maps made upon this principle, I have 



VOL. XIV, y 



