8 
CAPTAIX W. DE W. ABXEP ON THE TRANSMISSION 
In order to obtain from these measures a scale of the darkening obtained by 
arithmetical progression in the intensity of light, we have only to recollect that each 
hole admits ^2 times the light that the next smaller admits. By easy calculation 
this scale can be obtained, and is shown in the diagram. 
Fig. 8. 
I have shown elsewhere that the curves of blackness on platinum paper can be 
represented by the formula, A! = where is the amount of reflected white 
light, and A the amount reflected from the white paper, p being a coefficient, and x 
any power of 2, In this case, for curve I., p = ‘00302, and for curve II., p = ‘0103. 
XXIX. — Method of finding k. 
In the paper on Colour Photometry which appeared in the ‘ Phil. Trans.,’ General 
Testing and myself proved that a turbid medium prepared by dropping a solution of 
mastic dissolved in alcohol into water, obeyed Lord Batleigh’s law, as given above. 
If, therefore, one part of a piece of platinum paper were exposed for a certain time to 
sunlight after passing through a cell containing pure water, and a simultaneous 
exposure made on another portion of the jiaper with the same light after passing 
through turbid water prepared as above, the measures of the blackened paper would 
give the value of f, the photographic coefficient of absorption in the formula 
I^ = where y is the thickness of the turbid medium in any unit we please. 
Further, if the optical values of the light transmitted were compared when passing- 
through the same media, p, could be found and the value of y be reduced to atmo- 
