SCIENTIFIC DIAGRAMS 413 



fixed without exposure, answers exceedingly well for ink 

 drawings, and is convenient for those who have photographic 

 material at hand. 



Or the diagram may be simply scratched on a sheet of 

 gelatine about as thick as thin card, with steel needle-points 

 of appropriate fineness. This is a very convenient process, 

 because the gelatine may be fixed with drawing-pins over a 

 diagram to be copied, and can be thus traced exactly. When 

 finished, a little fine lamp-black, or black-lead, gently rubbed 

 over the lines, will make them a good black. Care need 

 only be taken to discard pieces already soiled or scratched, 

 and to keep the gelatine clean. Gelatine flowed on glass 

 and dried may be treated in the same way. 



For more finished and permanent diagrams, the best 

 method is undoubtedly one largely practised by Dr. Dallinger, 

 the drawing being made on very finely-ground glass, such as 

 is used for the focussing-glasses of the best cameras. For 

 some work a still finer glass called ' matted ' glass, etched 

 with vapour of hydrofluoric acid, is better even than this, 

 but is sometimes difficult to procure. Dr. Dallinger made 

 many exquisite drawings with black-lead pencil alone, which 

 is capable of most delicate and yet solid shading if the glass 

 is fine enough. The main secret is to use the very hardest 

 pencils, except for shading, when as soft as H B may be used, 

 and to keep a very fine point. The points are cut very long 

 to begin with, by a sharp knife; dressed up on a piece of 

 emery paper or rough ground-glass; and kept constantly 

 dressed sharp while in use, by an occasional rub on a piece 

 of finer ground-glass kept at the side. Close and repeated 

 working over gives the solidity of shading, but black should 

 not be attempted. If very black outlines are wanted, they 

 should be put in with Indian* ink used in a fine and hard 

 mapping-pen, or, better still, with a fine sable brush. 

 The latter will put in lines as fine as a hair, and I have seen 

 exquisite drawings of Floscularia made by Mr. Underbill in 



