152 Royal Society : — 



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 darkened mica is withdrawn from, and brought back over, a horizontal 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

 to the Table. If we wish to determine which of these shades 

 corresponds to another tint produced by a separate insolation, we 

 cannot make the comparison by daylight or ordinary lamp-light, as 

 these lights produce considerable changes of tint in the sensitive paper. 

 The two shades may, however, be perfectly and safely compared by 

 the light of a bright soda-flame ; this light possesses the great 

 advantage of being chemically inactive, and likewise of rendering 

 imperceptible those slight differences of colour which make the 

 comparison of two shades by the ordinary light so difficult. 



In order to compare any other photographic tint with the point of 

 equal shade on a strip, the latter, together with its millimetre scale, 

 is attached to a board, in a darkened room. The board slides in 

 a groove, so that it can be moved horizontally; and in front of the 

 paper strip a small block holds in a fixed position a small piece of 

 the tinted paper which it is desired to compare. On throwing the 

 light of a bright soda-flame upon both surfaces it is easy, by moving 

 the board from side to side, to find the exact point at which the 

 shade of the strip is identical with that of the other tinted paper. It 



