S. L. Penfield — Crystal Drawing. 



59 



an angle of 18° 26', but this is a condition to which one would 

 soon become accustomed. If orthographic and clinographic 

 projections are to be used together for purposes of illustration, 

 it is believed that the orthographic projections should be left 

 in the position in which they were drawn, and printed as in 

 figure 29, although this is a matter which need not be insisted 

 upon. 



Stereoscopic Effect. — It has frequently been observed that 

 the figures in text-books do not convey to many students the 

 impression of solidity, and this is a defect which probably has 



been generally recognized. Some have sought to overcome 

 the difficulty by making use of two projections drawn at 

 slightly different angles, as a crystal would appear if seen from 

 the positions of the right and left eyes, and then viewing the 

 two pictures with a stereoscope. The effects produced are 

 most satisfactory, but for purposes of text-book illustration 

 and for class-room work the method is scarcely practical. If 

 a clinographic projection is well drawn, with the front edges 

 represented by full lines and the back edges by somewhat 

 lighter, dashed lines, a very satisfactory and at times quite 

 remarkable stereoscopic effect may be had by viewing the fig- 

 ure through a tube. The practice is one commonly employed 

 by artists in studying effects. The tube may be a roll of light 

 or dark paper, either cylindrical or conical, quite variable in 

 size (6 cm long by l cm diameter gives good results), while the 

 most convenient thing to use is one's hand, doubled up so as 



