1862.] 309 



points of equal shade give the intensities of the light expressed in 

 terms of the above unit. 



This method of measurement is available only 



1 . If the phenomena of photochemical induction do not interfere 

 with the blackening of the paper. 



2. If a photographic surface of a constant degree of sensitiveness 

 can be prepared. 



3. If an unchangeable tint can be obtained which can be exactly 

 compared with the photographic paper. 



The result of a series of experiments made by varying the number 

 of the vibrations and calculating the intensity from each observation, 

 showed that photochemical induction does not exert any prejudicial 

 effect upon the measurements. 



The question into which the authors enter at greatest length as 

 being the most important for determining the exactitude of the 

 measurements, relates to the mode of preparing a standard paper 

 possessing a constant degree of sensitiveness. The relative degree of 

 sensitiveness is determined by exposing the papers to one and the 

 same light for the same length of time, and then comparing their 

 tints with the shades of a strip prepared in the pendulum-apparatus, 

 fixed in a solution of hyposulphite of soda, and furnished with an 

 arbitrary scale. The influence of the strength of the nitrate-of- silver 

 solution upon the sensitiveness is first examined ; a series of experi- 

 ments shows that with the same homogeneously salted paper, the 

 sensitiveness of the film does not alter when the strength of the silver 

 solution varies from 8 to 10 or 12 parts of nitrate of silver to 100 of 

 water. Further examination showed that the time during which the 

 paper lies upon the surface of the silver bath may vary from 15 seconds 

 to 8 minutes, without any difference in the sensitiveness of the paper 

 being noticed ; and no difference is found by the employment of 

 silver solutions which had been long in use and those freshly pre- 

 pared. The papers thus silvered may be preserved for from 1 2 to 

 15 hours in the dark without undergoing any change in their sensi- 

 tiveness. 



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 



