324 Proceedings of the Royal Physical Society. 



work can be done in Crystallography by means of simpler 

 methods than have usually been employed. I therefore pro- 

 pose in this and some succeeding papers to describe my own 

 mode of procedure, in language as simple as the nature of 

 the subject will admit, under the firm conviction that, if these 

 methods are mastered and followed, they will enable others 

 to do much practical work in Crystallography of a far higher 

 order than those who are accustomed to only purely mathe- 

 matical methods may at first be induced to believe. 



3. One of the first needs in practical determinative work 

 in Crystallography is a thoroughly good set of maps, and a 

 full knowledge of both the principles of their construction 

 and of their capabilities. Provided with these essential 

 requisites, with one or two good platyscopic lenses 

 (Browning's), with an ability to estimate both the relative 

 lengths and the directions of straight lines, and with the 

 faculty of delineating those lengths and directions correctly 

 by freehand drawing, the student of Crystallography may do 

 a large amount of good work of a practical nature without 

 having to use the goniometer, except in some few exceptional 

 cases, and without having to perform more than a few com- 

 putations that are of any other than a very simple nature. 



4. The principles upon which crystallographic maps are 

 constructed can be best comprehended by means of some 

 fair-sized actual crystal, say one of Barytes. This crystal is 

 to be placed at the centre of a hemispherical glass shade, 

 and then we have to imagine that lines are drawn from the 

 centre of the crystal perpendicular to its faces, and produced 

 until these lines touch the glass. There is not much prac- 

 tical difficulty in doing this with an approach to accuracy 

 sufficiently close for the purpose in view. The point where 

 these lines would touch the surface can easily be marked by 

 a dot of colour with the point of a fine brush. These points 

 represent the poles of the faces in question. 



5. If there happen to be several faces present on the 

 crystal under examination, it will soon be evident that the 

 poles of these faces are arranged in such a manner that three 

 or more than three are obviously in the same line. These 



