208 



PKACTICAL PHOTO-MICKOGKAPHY 



and ether will be sufficiently transparent, and it is necessary 

 to procure the collodion described as ' enamel collodion.' A 

 strong or even saturated solution of the dye in absolute alcohol 

 should be used say, one in a hundred and a few drops of this 

 added to a small quantity of the collodion and thoroughly mixed. 

 It may then be coated direct on to the glass plates as described for 

 the gelatin films, and allowed to dry. To ensure that the collodion 

 film adheres firmly to the glass an edging of a solution of pure 



india-rubber in benzole 

 may be applied as in the 

 wet collodion process if 

 need be. None of these 

 dry-film screens transmit 

 quite so much light as a 

 fluid screen, and for this 

 reason the latter is to 

 be recommended. 



A cell to contain the 

 solution of the dye, or 

 other liquid colour-screen, 

 may be improvised very 

 simply by taking two 

 pieces of stout glass, pre- 

 ferably thin patent plate, 

 placing between these a 

 piece of india-rubber tube 

 bent into horseshoe form, 

 or a flat india-rubber ring, 

 and clipping the whole 



together on either side by strong paper clips (Fig. 64 a). To 

 prevent bending of the glass, the clpis must engage well over 

 the edge so that the pressure is taken up evenly by the rubber 

 tube between. Such an improvised cell is perfectly efficient, 

 may be taken apart in a moment for cleansing purposes, and 

 its cost is but a few pence, so that a number may be kept at 

 hand ready for use. 



Some excellent cells for holding solutions may be obtained 

 from Messrs. Zeiss which are made entirely of glass (Fig. 65), 

 and are actually cemented and fused together, by what is known 

 as Leybold's process. The disadvantage with all cemented 



FIG. 65. Monochromatic Trough. 



