SCIENTIFIC DIAGRAMS 4" 



massive and costly the apparatus at hand, and the more fixed, 

 as it often is in large public institutions, the less will projec- 

 tion be needed, because more can be sufficiently seen without 

 it. The more cost, and portability, and storage have to be 

 studied, the more will this method come into play. On such 

 considerations I would refer again to the passage jsut cited. 



CHAPTER XXV 



SCIENTIFIC DIAGRAMS 



IT is no part of the intention of this work to enter into the 

 photographic production of slides for the lantern, which is of 

 course the best method of reproducing any printed diagram 

 suitable for the subject, or reducing any carefully made draw- 

 ing upon white paper or card. Such drawings should be 

 made in black Indian ink, care being taken to make any 

 lettering or shading as even as possible, when a reduced photo- 

 graph is certain to have a good effect. But ordinary diagrams 

 may readily be prepared by much simpler means. 



245. Drawings in White Outline. These are very effective 

 for suitable subjects, and easily prepared. A glass plate 

 should be warmed, and whilst warm rubbed with a piece of 

 paraffin, which should then be nearly wiped off again. Then 

 hold the plate over the smoke of burning camphor till suf- 

 ficiently black. On this black ground simple straight line or 

 free-hand diagrams are readily traced, but the plate should be 

 dropped into a sort of little socket or well slightly below the 

 surface on which the hand rests, that a straight-edge may be 

 held across it without touching. 



When curves or circles have to be struck, it is better to 

 cover the glass with an even coat of photographer's black 

 varnish, or, still better, the black varnish used by slide-painters. 

 The drawing can be best executed before the varnish has 



