412 OPTICAL PROJECTION 



become so dry as to be chippy. A small bit of horn should 

 be laid on the surface to support the leg of a pair of compasses, 

 and points should be provided of different fineness. Any little 

 injury to the black ground can easily be touched up last of all. 



There is however an objection to an opaque blacli ground 

 for diagrams, that a pointer can scarcely be seen upon it. 

 This objection may be removed, and the brilliance of white- 

 line diagrams still secured, by using for the ground a thin 

 film of blue printer's ink, laid on evenly with a dabber. The 

 lines will appear quite as distinct upon this, while the pointer 

 can be seen quite well upon the blue ground. 



246. Drawings in Black. Processes for making these are 

 very numerous. Some employ a film of sugar solution, or 

 of photographer's ' mat ' varnish, upon which it is said pencil 

 drawings can be made, but I have never found these very 

 satisfactory, and they are troublesome. The following are very 

 simple, easy, and effectual. 



For ink drawings, take a clean glass square, simply lick 

 it all over with the clean tongue, and let it dry in the air. 

 The imperceptible film of saliva thus communicated will 

 entirely prevent any ink from ' spreading ' on the glass from 

 a fine pen-point, the best of which are the crow-quill steel 

 pens sold for map -drawing. The plate may be laid over any 

 drawing, and a copy traced, or any free or ruled drawing may 

 be made, with common writing-ink in which a lump of sugar 

 is dissolved, or any of the syrupy ' copying ' inks. These 

 lines are not sufficiently black ; but when dry, if a little finely 

 powdered lampblack be rubbed lightly over them with a 

 morsel of cotton- wool, sufficient will adhere to the sticky 

 lines to make an opaque drawing. 



Those who prefer, and are skilful with, a fine sable or 

 camel-hair pencil, will find Indian ink take ' perfectly well 

 on a glass plate over which has been flowed water containing 

 a very small quantity of gelatine. 



A photographic dry plate chloride is better than bromide 



