DELINEATION OF CURVED SURFACES 307 



superposed, the curves exhibit the proper relations in space 

 and afford a very fair idea of the nature of the surface of which 

 they are elements, without arousing any sensation of an in- 

 cluded volume. Since the lines can always be erased and re- 

 placed by others, a set of twenty plates suffices for all pur- 

 poses, and the surfaces can be produced at a moment's notice 

 if the necessary sketches on paper are preserved. Besides 

 [264] being useful for illustrating lectures in molecular phys- 

 ics, the plates can also be employed to advantage in the con- 

 struction of crystallographic, geological and other models, j 

 Where the parallax, inevitable for glass plates, becomes 

 annoying, it is possible to substitute wide-meshed cotton 

 netting, stretched upon square frames of uniform thickness. 

 The curves can be embroidered upon the net, as it were, with 

 pieces of colored thread; although it is not quite so easy to 

 make the lines conform to the drawing, the general effect 

 remains the same. 



