jS. L. Penfield — Crystal Drawing. 



61 



Uniformity of Lettering. — A gain has been made in recent 

 years in adopting some uniformity in lettering, as must be 

 appreciated by all who are accustomed to use Dana's System 

 of Mineralogy. The scheme there adopted is in general to 

 indicate the three pinacoids, 100, 010 and 001, by a, b and c, 

 respectively, and the prism 110 by m. In the hexagonal and 

 rhombohedral systems the prisms of the first and second orders 

 are designated by m and a, respectively, in conformity with 

 the usages of the tetragonal system, and the unit rhombohe- 

 dron is designated by r. In the isometric system the cube, 

 octahedron and dodecahedron are lettered a, o and d, respec- 



tively. The writer recommends going still one step further 

 and designating the form 111 (isometric system excepted) 

 always by p, but to carry the scheme beyond this point would 

 be cumbersome and scarcely practicable. In using Dana's 

 Mineralogy, or reading any article in which the scheme as 

 outlined above is followed, a glance at the figures will gen- 

 erally serve to indicate the character of the forms, for however 

 complicated a crystal may be, it is almost certain that some of 

 the above mentioned forms will be present. It is hoped that 

 the scheme will be more generally adopted than it is at present. 

 Examples. — In conclusion some figures will be given illus- 

 trating numerous advantages derived from drawing crystals in 

 both orthographic and clinographic projection. 



