158 Laboratory Arts 



light, the better will be the negative secured, but at the expense of 

 shortness of exposure. A background of neutral tint grey or 

 purple-grey composed of a ground colour painted on stretched 

 canvas mounted upon a wooden frame, should be provided. 



The colour may be prepared as follows : Take 2 Ibs. of " ball" 

 whiting, mix with half a gallon of water, and stir till no lumps 

 remain. Add to this 8 ozs. of glue, soaked in cold water for 

 twenty-four hours previously to use, and melted in the soaking water 

 (which should only just cover the glue) by immersion in a pan of 

 boiling water. Stir well and add I oz. of ultramarine, finely 

 powdered, and enough red ochre or sesquioxide of iron to " kill " 

 the blue colour and produce a neutral grey tint. Lampblack 

 ground in water containing a little methylated spirit may be added 

 to darken the shade if necessary. The whole preparation is then 



FIG. 116. Lantern slide making apparatus. 



well stirred, and applied with a coarse brush, such as a whitewash 

 brush, to the stretched canvas, the spaces in which will be filled up 

 by the mixture. When the first is dry, a second coat, containing 

 slightly more water than the above preparation, may be given. 

 All materials must be finely powdered, and ground with water 

 before mixing. 



The apparatus, mounted in front of this neutral background, is 

 then photographed in the usual way, using a small stop, and giving 

 a long exposure. 



Diagrams or book illustrations may be photographed similarly, 

 while supported between two retort stands, and held in position by 

 the clamps, in which case the burning of 3 inches of Mg ribbon 

 on each side the lens of the camera is the most satisfactory method 

 of illumination. Messrs. Reynolds and Branson, Ltd., of Leeds 

 supply an exceedingly convenient apparatus for the photography of 



