138 



jS. L. Penfield — Stereographic Projection. 



may be employed. The construction of the instrument is very 

 simple. A strip of strong, straight-grained wood, a a, is bent 

 by pressure applied in one direction at points pp, and in oppo- 

 site direction at points r r. Pressure is applied by means of 



tff^r* 



the lever Z, which is held in any desired position by means of 

 the nut JV. The instrument may be quickly adjusted, and the 

 curve between the points a a, even when the bending is con- 

 siderable, corresponds exactly with the arc of a circle. It is 

 no drawback that the wooden strip a a becomes permanently 

 bent when left for some time under pressure in the instrument. 

 It is only necessary to withdraw the strip and reverse it, in 

 order to draw a very flat arc, almost a straight line. A curved 

 ruler, in principle like the one figured, but made of metal, was 

 first described by W ulff .* Later Fedorow elaborated the instru- 

 ment somewhat.f The writer has a metal instrument, made 

 after the Fedorow model by Fuess of Steglitz, near Berlin, but 

 has not found it as satisfactory as the one made of wood. 

 This, however, is only because of certain minor defects. The 

 metal strip in the Fuess instrument, corresponding to a a of 

 figure 31, is so high that it is difficult to use a pencil, and 

 almost impossible to use a ruling-pen, in contact with it. More- 

 over, it is so highly polished that reflections from it are very 

 annoying. 



32 



lllljiliil iiiSijillij |!ll!jli!l| |lill|l!ll| |!!!l|l!ll| |lll!|l 



Figure 32, which may be designated as the scale of decimal 

 parts, gives a series of spaces (millimeters as indicated by the 



* Zeitschr. fur Kryst., xxi, p. 253, 1892. 

 f Zeitschr. fur 'Kryst., xxi, p. 618, 1893. 



