310 



[Dec. 11, 



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 

 examined. Experiment shows that differences in the thickness of 

 white paper, such as is usually employed for photographic purposes, 

 is without 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. 



