206 PKACTICAL PHOTO-MICKOGKAPHY 



with comparatively little trouble. The matter may be dealt 

 with in either of two ways. Permanent screens may be made 

 by dyeing either gelatin or collodion films ; or, what is much 

 better, liquid screens may be employed, and these may be 

 adapted either in their general or selective absorptive powers 

 to the requirements of the case. 



The method of making permanent screens is fairly simple, 

 although perhaps the term ' permanent ' is rather misleading, 

 as nearly all aniline dyes are subject to change on exposure 

 to light for any long periods. Whether the screen is made 

 of collodion or gelatin will depend on the dye, which may be 

 soluble either in Water or alcohol. If soluble in water, the 

 gelatin film may be made in either of two ways. Some of the 

 best hard gelatin, such for instance as Nelson's No. 1, may be 

 made up into a 7J % solution in water. If the screen is being 

 made in very hot weather, then it is advisable to make the 

 gelatin solution as strong as 10 % to ensure its setting easily. 

 Take a sufficient quantity of the gelatin, and add water to the 

 strength stated : allow the gelatin to soak in the water for some 

 hours, and then place it over a water -bath until it is completely 

 dissolved. The gelatin may now be filtered through glass-wool 

 or cotton-wool and coated on to small pieces of glass, con- 

 veniently about three inches square. This is usually done 

 by pouring on to the centre of the glass a pool of the hot gelatin 

 solution, so that about 2 or 3 c.c. of solution are allowed to 

 each piece of the size stated ; with a piece of glass rod at once 

 cause the gelatin solution to flow evenly over the glass, and 

 place it to set in a position free from dust and on a perfectly 

 level surface. As soon as it is set, it may be immersed in a 

 solution of the required dye, until it is stained sufficiently deep 

 for the purpose in view. The exact strength of the dyeing- 

 solution is immaterial ; the stronger it is, the shorter the time 

 necessary to effect the desired staining. On the other hand, too 

 strong a solution of the dye will often stain too rapidly, so that 

 it is difficult to stop exactly at the right moment. In general 

 with aniline dyes, a strength of one in one thousand will be 

 sufficient. On the completion of the dyeing process, they 

 should be taken out and quickly rinsed in distilled water, then 

 set on edge or in a photographic drying-rack to dry. 



An alternative method i& to add the dye-solution directly to 



