154 



Royal Society. 



pared. The papers thus silvered may be preservedYor from 12 to 15 

 hours in the dark without undergoing any change in their sensitiveness. 

 If the paper be allowed to float on the surface of the solution of 

 chloride of sodium as on that of the silver bath, the sheet after silver- 

 ing exhibits, on drying, a very unequal degree of sensitiveness in its 

 various parts. If, on the contrary, the sheet be well soaked in the 

 salt-bath no such irregularity appears, and the sheet is of an equal 

 degree of sensitiveness throughout its whole surface. This fact is 

 determined by several extended series of experiments. The effect of 

 change of concentration of the salt-bath upon the sensitiveness of 

 the film is very great ; and, as far as the observations extend, no limit 

 exists beyond which an increase or a diminution of the percentage 

 of salt in solution ceases to affect the sensitiveness of the film. 

 Hence, in order to obtain constant results it is necessary to employ 

 a solution of chloride of sodium of constant strength. By using 

 solutions of the same strength, papers of a constant degree of sensi- 

 tiveness are obtained. 



The influence of the thickness of the paper employed is next, ex- 

 amined. Experiment shows that differences in the thickness of white 

 paper, such as is usually employed for photographic purposes, is with- 

 out influence upon the sensitiveness of the film of chloride of silver. 

 The changes in atmospheric temperature, from 3° C. to 50° C, 

 and in atmospheric moisture are likewise found not to influence the 

 sensitiveness of the prepared paper. 



From the experimental results detailed in the communication, it 

 appears that by adhering to a certain mode of preparation, a standard 

 paper can be obtained, which at all times possesses a degree of sensi- 

 tiveness sufficiently constant for the purposes of exact measure- 

 ment. In the following extract from a larger Table, the readings are 

 given which were made from papers prepared in three different salt 

 solutions of the strengths mentioned, and silvered in a solution con- 

 taining 12 of nitrate of silver to 100 of water. Equality in the 

 numbers in each of the columns III. and IV. denotes equality in the 

 readings and in the tint, and therefore equality in the sensitiveness 

 of the prepared surfaces. Three sheets of paper were dipped into 

 each solution. These numbers likewise show the great degree of 

 accuracy with which tints can thus be compared. • 



I. 



Paper. 



II. 



N a CI to 100 parts 

 of water. 



III. 



Intensity 

 No. 1. 



IV. 



Intensity 

 No. 2. 



Upper part of sheet No. 2 ... 

 Middle part of sheet No. 3 ... 

 Middle part of sheet No. 2 ... 

 Lower part of sheet No. 2 ... 



3-026 

 2 950 

 3-028 

 3000 



87-0 

 86-3 

 86-0 

 85-9 



75-4 

 74-4 

 74-9 

 74-4 



The next subject considered is the preparation of an unvarying 

 tint which can be easily obtained and used as the standard of com- 

 parison. This is effected by grinding together 1000 parts of pure 

 oxide of zinc with 1 part of pure lamp-black. A series of experi- 

 ments showed that a colour can thus be prepared which possesses a 



