CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
266 
Taking 532 and 287 as lying on the curve derived from T = Ie - ^, the values 
obtained were as follows— 
532, 456, 391, 335, 287; 
arid the value for the coefficient of transmission was = '862, a value somewhat lower 
than that obtained for the normal-eyed person, though not very far from it. 
I think in the foregoing enough has been adduced to show that, whether the loss of 
intensity by different rays in passing through the atmosphere obeys Lord Rayleigh’s 
law or follows the observations made by Langley, astronomers have been quite 
warranted in using the logarithmic law, at all events in observations made to 75° 
zenith distance. 
§ XV. Photographic Values of the Integral Value of Light through different 
Air- Thicknesses. 
We thus see that as far as light, qud light, is concerned the method of observing the 
integration of the different wave-lengths gives results concordant with those obtained 
by treating the light analytically. The point that next presented itself was as to 
whether the photographic values of radiation when treated in the same manner 
would give a similarly satisfactory result. If so, then the photographic values of 
sunlight which I had obtained could be applied to the question of total value of sun¬ 
light on any particular day. 
In the ‘ Proceedings of the Royal Society ’ I have shown the photographic 
sensibility of a particular salt of silver for the different rays of the spectrum; 
and, knowing the day and hour on which that value was determined, and assuming 
the minimum value of the coefficient for scattering as applicable, after constructing 
the necessary curves and taking the areas, the following results were obtained at 
1, 2, 3, and 4 atmospheres. The values of areas of the curves were— 
621, 457, 340, 253, 
and those derived from using the formula V — Ie^k g being "2993 and a '74, were— 
621, 461, 342, and 253. 
The maximum value of the spectrum on this salt of silver lies well in blue, and it 
was thought that a theoretical consideration of the value when the photograph was 
taken on a salt of silver which had a maximum in the extreme violet would be useful, 
since such a salt—the chloride—is used in Roscoe’s actinometer. The spectrum 
value of this salt had been previously carefully taken by myself, and the values were 
applied to each unit of my scale. The values obtained for 0, 1, 2, 3, and 4 atmospheres 
from the curves were as follow— 
690, 
413, 
246, 
147, 
87; 
