60 6*. L. Penfield — Crystal Drawing. 



to form a sort of tube. Stereoscopic effects are more pro- 

 nounced with some figures than with others, but they would 

 seem to depend to a large extent upon the proper proportion- 

 ing of the heavier front and lighter (dashed) back lines. It is 

 believed that the reason for the stereoscopic effect is not far 

 to seek ; — it seems to be wholly an optical illusion. — By look- 

 ing through a tube the attention is concentrated on a single 

 figure, and the heavy lines produce the effect of being near, 

 the fainter, dashed lines of being farther away ; hence the 

 conception of solidity. In order that the stereoscopic effect 

 may be observed by the reader, illustrations of three crystals 

 are given for comparison, drawn with and without dashed back 

 lines ; Figure 30 is a combination of dodecahedron, d, and 

 octahedron, o, magnetite ; figure 31 is a combination of prisms 

 of the first and second order, m and a, terminated by pyramid 

 and base, vesuvianite ; and figure 32 is a combination of tri- 

 clinic forms observed on axinite. Except for stereoscopic 



33 



effect it may be questioned whether dashed back lines are not 

 at times as much of a hindrance as a help in the understanding 

 of crystal figures, because of the complexities which they intro- 

 duce. As a rule they certainly add to the effectiveness of a 

 figure, but not always ; for example, in figure 18, page 50, it 

 seemed far better to do without them. 



Size of Original Drawings ; Lettering. — Generally speak- 

 ing, the size of an original drawing should depend to a large 

 extent upon the complexity of the figure. It may be recom- 

 mended to draw simple figures three or four times as large as 

 needed for illustration, while with a complex subject like figure 

 34 it is almost impossible to make a drawing with accuracy 

 except on a scale seven or eight times the size of the illustra- 

 tion. Figure 34 represents a crystal with 240 edges ; hence it 

 is evident that it is necessary to make the original drawing on 

 a large scale in order to preserve with accuracy the directions 

 of the many short lines. 



If figures are to be reduced by the photo-engraving process, 

 they must be drawn in ink and lettered to suit the reduction. 

 Figure 33 gives the approximate width of line and size of letter 

 to be used with various degrees of reduction indicated by the 

 numbers. Almost any one can succeed fairly well in forming 

 letters who will take pains and make use of good models. 



