HALATION 219 



does not prevent the spreading of light in the film, but it 

 does cause a decided improvement in the general bright- 

 ness of the negative. 



Films and Backed Plates. The thinner the glass the 

 nearer the secondary images approximate to the central 

 one, and on films they are practically coincident. 

 Although films have this great advantage over plates 

 and are much more convenient for storage, they are not 

 so satisfactory on the whole, and most workers find it 

 easier to get good results on plates. 



Backed plates should always be used in order to avoid 

 halation as much as possible. The backing takes several 

 forms. A soluble coating of caramel is very satisfactory 

 and need not be removed before development, as it 

 dissolves in the solution ; the developing solution 

 becomes black, but if only used for one or tAvo plates no 

 harm results. Some makers mix insoluble black or red 

 pigments in the backing material, in which case it should 

 be removed with a damp sponge before development, 

 otherwise pieces ma}^ float on to the surface of the film, 

 from whence they are difficult to dislodge. It is usual to 

 buy plates ready backed, but the glass side of unbacked 

 plates can easily be coated with caramel, which is 

 specially sold for that purpose. If too thick to spread 

 evenly the syrup should be thinned somewhat with 

 alcohol, and rubbed on the back of the plate with a glass 

 rod over which a piece of rubber tubing has been passed. 

 The rod is conveniently shaped like a D, the tubing 

 covering the straight side. A very thin layer of backing 

 only is required, and this soon dries ; but if the plate is 

 wanted immediately the backing should be covered with 

 a piece of paper before placing in the dark slide. It is 

 very difficult to back panchromatic plates on account 

 of the necessity for working in the dark, and they should 

 always be bought ready backed. An anti-halation film 

 between emulsion and glass is adopted in the Chromo- 



