308 [Dec. 11, 



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, diiferences 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 to that of the other tinted paper. It 

 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 ; 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 



