248 PEACTICAL PHOTO-MICEOGKAPHY 



a technically good negative. In gelatin plates this is also 

 possible up to a certain point, but the slide is likely to suffer 

 much more from stain and loss of gradation than in wet 

 collodion. Intensification is effected by applying to the 

 slide, after fixation and washing, the following solution : 



Pyrogallic acid .... 1 gramme. 

 Citric acid .... 2 grammes. 

 Distilled water .... 250 c.c. 



Just before applying this solution to the plate, a few drops 

 of a five-per-cent. solution of silver nitrate in distilled water 

 may be added; but this should only be done immediately 

 before use, and to such a quantity of the solution as will be 

 used at once. The solution is poured over the plate exactly in 

 the same manner as in development, and the treatment is 

 continued until sufficient density is obtained. Should the 

 solution become turbid before completion, then it must be 

 washed off and a fresh portion applied. The plate is finally 

 washed under a gentle stream of water, and dried. 



In pictorial photography it is usual to tone collodion 

 slides, as the colour is often not all that might be desired. 

 For the purpose under discussion, however, there would not 

 generally appear to be any necessity to go to this additional 

 trouble, since, providing the slide is of a suitable black colour, 

 such after-treatment is unnecessary. 



In case the slide may not have acquired such a black colour 

 by development, it may be altered by toning in a solution of 

 chloride of gold in distilled water. The strength may con- 

 veniently be one grain of gold chloride to ten ounces of water. 

 As the film of a collodion slide is much more liable to injury 

 than gelatin, it is advisable to varnish it. Any clear colourless 

 negative-varnish may be used. 



In the formulae given in this section, the decimal system 

 has as far as possible been adhered to ; but in one or two cases, 

 as a matter of convenience, this has been departed from. 

 For instance, it is universally the custom to take one ounce of 

 developing-solution as the normal quantity for a quarter-plate ; 

 a departure in such a case, therefore, would not appear to be 

 without justification. 



A few words may be useful as to the method of projecting 



