PHOTOGEAPHIC PEOCESSES 245 



that absolute cleanliness is perhaps even more essential : any 

 small speck of dust or any imperfection in the negative becomes 

 so painfully evident when a slide is projected on the screen, that 

 it is of the utmost importance to ensure that no such blemish 

 shall be present. Probably all dry-plate lantern-slides can be 

 improved by a short immersion in the Howard Farmer reducing- 

 solution, already mentioned. This is not with a view to general 

 reduction of density, but to brighten up the image and to 

 remove any slight general fog that may be present. It is 

 also at times most useful to apply the solutions by means of 

 a tuft of cotton-wool. By this means local reduction may, 

 if necessary, be effected, and it is sometimes of the greatest 

 value in enabling a good slide to be produced from an 

 indifferent negative. 



The most perfect method of making lantern-slides is the 

 wet-collodion process, and, as this process lends itself to 

 negative-making, it will be briefly described. 



As already stated, the grain of the image in a collodion 

 process is finer than in a dry-plate ; consequently, for lantern- 

 slide work, where fine detail has to be shown, or for the highest 

 class of negative-making, it is still unrivalled. For a busy 

 worker there are considerable objections to it : the plate must 

 be prepared and coated before an exposure can be made, and 

 this entails a considerable amount of additional work as com- 

 pared with the dry-plate process. Lantern-slides thus made are, 

 however, particularly transparent and brilliant ; and the process 

 has the added advantage that where a slide has been made 

 from a thin negative and lacks in brilliancy it can be intensified 

 to an extent that is impossible with a dry-plate. The necessary 

 operations are briefly as follows. An ordinary 3J-inch-square 

 glass plate is thoroughly cleansed and coated with collodion. 

 The cleansing process must be carried out very efficiently, and, 

 if the glass has not been previously used, it must be well washed 

 in hydrochloric acid and water, followed by a thorough rinsing 

 in clean water. Before the plate is coated with the collodion, it 

 must be edged to prevent the film slipping off in the subse- 

 quent developing operations. This is done by running round 

 the outer edge of the plate, by means of a camel-hair brush, a 

 narrow edging, about one-sixteenth inch wide, of a one-per-cent. 

 solution of india-rubber in benzol, and this must be allowed to dry 



