Messrs. Bunsen and Roscoe's Photochemical Researches, 153 



is then only necessary to consult the Table in order to find the time 

 in seconds during which the paper must have been exposed in order 

 that it should attain the tint in question. A series of lights of known 

 intensities was obtained, by allowing the sun to shine through holes of 

 known size. The images thus formed fell on to a piece of prepared 

 paper j and the tints produced were compared with a strip darkened 

 in the pendulum-apparatus, and thus the time of exposure necessary 

 to effect the shade determined. Experiments made with intensities 

 varying from 1 to 50, show that within these limits equal shades of 

 blackness correspond to equal products of the intensities of the acting 

 light into the times of exposure ; so that the light 1 acting for the 

 time 50, produced the same degree of blackening as the light 50 

 acting for the time 1 . 



A method for measuring the chemical action of light by simple 

 observations is then founded upon this proposition. Thus, if we 

 assume as the unit of photochemical action that intensity of light 

 which produces in the unit of time a given degree of shade, we have 

 only to determine, on a strip of paper tinted in the pendulum- 

 apparatus, the point where the shade of the strip coincides with the 

 given tint ; the reciprocals of the times which correspond to these 

 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 serips 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- 



