14:4c 8. L. Pen field — Stereographic Projection. 



ing triangles will probably be limited, since an expensive 

 instrument must be procured, which not only must be kept in 

 perfect adjustment, but can be used only by persons who 

 thoroughly understand the workings of its several parts. 

 Then, too, the instrument does not give a map or chart, which 

 is such an important feature of the stereographic projection. 



Although it is not customary for the writer of a scientific 

 article to advertise, there are many who may wish to apply 

 stereographic methods to the solution of problems in which 

 they are especially interested, and therefore will find it con- 

 venient to make use of the printed sheets, scales, and protrac- 

 tors described herewith. Accordingly arrangements have been 

 made to keep a supply of the necessary articles in stock, and, 

 upon application either to the writer or to E. L. Washburn & 

 Co. of New Haven, a price list will be sent. 



If the method fulfills the writer's expectations, there will be 

 in time a demand for scales and protractors of larger size and 

 finer graduation, based, for example, on circles of 12 or 18 

 inches diameter. Moreover, with the experience already gained, 

 there ought to be no serious difficulty in making them very 

 accurately. 



Sheffield Laboratory of Mineralogy and Petrography, 



Yale University, New Haven, Conn., December, 1900. 



