324 OUR PHYSICAL WORLD 



Sometimes a negative or a lantern slide is too thick or too thin 

 when finished to give satisfactory results. Such may be improved 

 by reducing or intensifying. To reduce, add a teaspoonful of a 

 saturated cold-water solution of potassium ferricyanide to a 

 solution of hyposulphite of soda made by adding a tablespoonful 

 of this salt to 4 ounces of water. These proportions do not 

 need to be exact. Put this in a tray and lay the negative in it, 

 rocking the tray to cover all parts promptly. The operation 

 is carried on in daylight. The more of the ferricyanide used, 

 the more rapid the reduction. The negative is taken out when 

 sufficiently thin, washed in running water twenty minutes, and 

 set up to dry. 



To intensify, place enough saturated cold-water solution of 

 bichloride of mercury (poison) to cover the plate in one tray, 

 and a similar amount of water to which ten drops of concentrated 

 ammonia are added in a second tray. Immerse the plate in the 

 first and leave until its surface whitens a bit. Then put it in 

 the second tray where it will darken, especially the more opaque 

 areas. Wash it in water for two or three minutes, and repeat 

 the process until it is sufficiently intense. Then wash twenty 

 minutes in running water and dry. There are many other 

 methods of reduction and intensification that use other chemicals; 

 these methods will be found in the books given in the Book List. 



The sensitive film or plate or paper is produced by spreading 

 evenly on these objects a thin layer of gelatine all through which 

 there are suspended tiny particles of silver bromide and silver 

 iodide, put there by apparently dissolving these salts in the 

 gelatine. If the preparation is made up hot and allowed to 

 stand and ripen before it is spread, the particles of silver salt 

 aggregate somewhat, and the plate is coarse grained, but rapid. 

 If it is made up cold and spread at once, the particles do not 

 cohere, and the plate is slow, but fine grained. The slow plate, 

 therefore, will give finer detail than a rapid one. 



