102 THE BOOK OF THE LANTERN. 



latter over the former with a steady circular motion, taking 

 care to coyer every part in turn. This rubbing should 

 be <co*itjjnud for about ten minutes, adding water if the 

 two, surfaces aeem inclined to stick together too much, and 

 occasionally -collecting with a knife-blade the mud which 

 oozes out between them, and putting it once more in the 

 centre of the under glass. At the end of the time 

 named* the glass can be lifted from the table, held under 

 the tap for a few seconds so as to clean it, and care- 

 fully examined by transmitted light. It will most probably 

 show a fine grain, except in certain parts, which remain 

 clear as before. These clear portions are depressions in 

 the surface of the glass, which the emery has failed to 

 reach. The grinding operation must be repeated as before 

 until on examination these clear places have disappeared. 



It has occurred to me that Dr. Dallinger's system of 

 producing lantern-slides might be modified with advantage 

 in the following manner. Coat the glass with varnish 

 which dries with a mat surface, and practically gives the 

 same effect as ground-glass itself. There are several 

 recipes for such varnish, which is used by photographers for 

 retouching purposes. Here is one which will be found 



effectual. 



Ground- Glass Varnish. 



Sandarac ... ... ... 90 grains. 



Mastic 20 



Ether - ... 2 oz. 



Benzole Jtoljoz. 



The proportion of benzole added determines the nature of 

 the mat obtained. 



