1862.] 307 



In spite of the various futile attempts which have been made to 

 register and measure the chemical action of light by means of photo- 

 graphic tints, it still appeared possible in this way to attain the 

 desired end. No instruments founded on such a mode of measure- 

 ment can yield reliable results unless we know the conditions under 

 which photographic surfaces of a constant degree of sensitiveness 

 can be obtained, and unless the relations be determined which exist 

 between the degree of tint produced, and the time and intensity of 

 the light acting to effect such a tint. 



The first point which the authors examine, is whether the photo- 

 graphic tints produced vary in shade in the direct ratio of the 

 intensities of the acting light. Several experiments proved that no 

 direct ratio between the degree of blackening and the intensities of 

 the light exists. Hence it is necessary to relinquish the idea of 

 employing any mode of measurement founded on the comparison of 

 photographic tints of different shades. The next point examined is 

 whether equal shades of blackness always correspond to equal pro- 

 ducts of the intensities of the acting light into the times of insolation. 

 For the purpose of testing the truth of this proposition, an instru- 

 ment is employed by which photographic sensitized paper can be 

 exposed for times which can be exactly measured to within small 

 fractions of a second. This instrument consists essentially of a 

 pendulum vibrating about f- seconds, by whose oscillation a sheet of 

 darkened mica is withdrawn from, and brought back over, a horizontal 

 strip of paper prepared with chloride of silver, and fixed in a con- 

 stant position relative to the pendulum and sheet of mica. The time 

 during which each point in the length of the strip is exposed is 

 different, and the time of insolation for each point can be calculated 

 when the length and position of the strip, and the duration and 

 amplitude of the pendulum's vibration are given. A Table exhibits 

 for each millimetre in length of the strip, as measured by a scale 

 attached, the time of exposure in seconds which the corresponding 

 point of the strip undergoes in one vibration of the pendulum. 

 These numbers require to be multiplied by n if the paper has been 

 insolated for n vibrations. 



The paper insolated whilst the pendulum is oscillating, exhibits 

 throughout its length a regularly diminishing shade from dark to 

 white ; and the time of insolation of any point is found by reference 



