Simpler Methods in Grystallogra'phy. 433 



the directions of the lines at first by freehand drawing from 

 the pattern ; and then to redraw it a third time by fixing the 

 sketch upon the pattern, and ruling the lines with a fine 

 pencil and any convenient arrangement for drawing one line 

 parallel to another. The ordinary parallel rulers are of 

 little or no use for this work. There is no plan so good as 

 that of using a rather-thick steel straight-edge and a good 

 vulcanite set-square. With practice this is far more con- 

 venient than any tee-square, or any other of the instru- 

 mental arrangements commonly in use amongst draughtsmeu. 

 It is well to make the drawing of a fair size, which is not 

 difficult to do by this method. It can easily enough be 

 reduced afterwards by photographic or other processes, or 

 even by proportional compasses. Hot-pressed paper or good 

 thin cardboard are best for the purpose. All the drawings I 

 have put out in connection with the Scottish Mineral Collec- 

 tion are on thin white card. 



How to Deal loitli Douhtfid or Undetermined Forms. — It must 

 frequently happen that some one circumstance or another 

 prevents the determination of some form or forms of a crystal 

 under examination. Perhaps it is in a deep cavity, and can- 

 not be got out to be measured; or its faces may be too dull, 

 or too uneven, to afford the requisite reflections, even when 

 coated with black lead or glazed with a thin coating of gum. 

 In such cases it may be possible to make a close approxima- 

 tion to its position, e.g., it may be known to occur in a 

 certain zone, in an undetermined position between two other 

 faces which have been identified. In such a case as this 

 latter, an asterisk placed beneath a bracket connecting the 

 symbols of the two known forms suffices for the purpose. 

 In other cases, where that kind of evidence fails one, and 

 - there is no means of identifying the face, a simple asterisk 

 will usually suffice. Where one has to deal with curved 

 faces which obviously bend over from one known face to 

 another also known, a simple dash between the two serves 

 the purpose of denoting the curvature. Thus t — y would 

 signify that there is a curve from what is known to be t 

 to another known to be y. Calcite and Gypsum furnish 

 abundant illustrations of what is meant. But with practice 



