118 



THE BOOK OF THE LANTERN. 



other words, " the intensity of illumination on a given sur- 

 face is inversely as the square of its distance from the source 

 of light" 



FIG. 39. 



Fig. 39 will perhaps make the matter still more plain. At 

 1 foot from the candle the square marked i receives a 

 certain amount of light ; at u, that light is spread over a 

 surface four times the area of the first ; and at in over a 

 surface nine times as large as I. This law, which is really 

 of a very simple character, the operator should have con- 

 stantly in his mind, as he exposes his plates, to artificial 

 light under a negative. 



The method of development which I recommend is by 

 means of ferrous oxalate, and it will be found that the 

 transparencies produced by- it require no toning whatever. 

 I make my own potassic oxalate, and find it, moreover, 

 reliable ; the operation is simplicity itself. In a large 

 basin dissolve half a pound of carbonate of potash (salts of 

 tartar) in a pint and a half of warm water. Now add 

 gradually oxalic acid, a few cr3 T stals at a time, for the 

 effervescence is very violent, and difficult to control if 

 much be put in at once. When six ounces of the crystals 



