140 



S. L. Penfield — Stereographic Projection. 



holder can be held vertical, and, on a diameter or arc previ- 

 ously drawn, a puncture can be made which is centered very 

 exactly with reference to the graduations of the scales and 

 protractors. A cheap lens is trying to the eyes, as it distorts 

 the lines of the scales, therefore persons making much use of 

 the stereographic projection will find it greatly to their advan- 

 tage to have a half-lens made from an aplanatic triplet. Figure 

 33 represents a half-lens made for the writer from a Hastings 

 triplet, two-inch focus, by the Bausch and Lomb Company of 

 Rochester, N. Y. This form of lens will doubtless be con- 

 venient for many kinds of work. 



34 



Black-hoard Demonstration of the Stereographic Projec- 

 tion. — In explaining the projection to an audience or class, it 

 may be convenient to make use of a black-board, and with a 

 few implements, demonstrations can be made very quickly and 

 accurately. 



Figure 34 represents the black-board equipment employed 

 by the writer, in about one fourteenth its natural size. The 

 black-board, AAA, is of slate, on which is scratched the circle 

 BB, with center G. The circle has a radius of 35 cm , and its 

 graduation gives every tenth degree. The circle and gradua- 

 tion marks need not be conspicuous, and they interfere in no 

 way with the ordinary uses of the black-board. In case a 

 permanent circle, scratched or painted on a black-board, is 

 objectionable, a center point may be fixed, and degrees on any 

 circle described about it may be taken from graduation marks 



