OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
273 
causes in diminishing the amount of transmitted light. Lord Rayleigh has shown 
that the sizes of the particles have a considerable effect in the scattering, and when 
we have white mist the scattering of light must be general rather than selective. 
But it may happen that the general aggregate of particles present may be of sizes 
which to a greater or less degree refuse to scatter light of wave-lengths not taken 
between certain limits. Or the sizes of the particles maybe so varied that, whilst one 
gives a limit of scattering for one certain wave-length, another may give a limit for 
another, and so on till the final outcome may be to give a loss of light not exactly 
varying as X -4 . This, it appears to me, may be the meaning of this law being obeyed 
on days which are perceptibly misty, as in some November and December days, and 
in which the integration method by photography is not in accord with the optical 
method adopted. On the whole, I am inclined to think that on fine days, near mid¬ 
day, with pure blue sky, the water particles are present in numbers, and, if dust 
be fairly absent, that then we get the minimum loss. At night, when the tem¬ 
perature is diminished, the water vapour probably condenses to give a larger number 
of small water particles, and hence star observations give a greater value for the 
coefficient of transmission than I have obtained for the minimum, though the mean 
value they have deduced is not far from the value I obtain when the coefficient kx is 
equal to '0019, which is a value that on several occasions I obtained. That these 
water particles have much to say to the coefficient of transmission is shown by 
Pritchard’s determinations at Oxford and Cairo respectively ; the former gave 
a coefficient of '791 and the latter "841. 
§ XXI. Deductions from the Riff el Observations. 
It would be premature to deduce too much from the observations taken at the 
Riffel. It will be seen that the air-thickness at the Riffel at noon on the day observed 
is equivalent to 1 atmosphere at sea-level, and that our equivalent value is really a 
good deal less than that thickness. This means that there are at higher altitudes 
proportionally fewer particles to scatter the light than at sea-level. I refrain from 
giving the values I obtained near sunset at the same place, but the value of absorption 
I found to be startlingly smaller, so much so that my results must be repeated before 
I can vouch for the deductions to be made. It seems to me that in the Alps we have 
the most favourable conditions for studying the atmospheric permeability for light, 
cwdng, in proper seasons of the year, to the absence of dust. It should he pointed 
out that the radiations which act on our eyes as light are less absorbed by 
aqueous vapour than are those radiations which lie in the infra-red, and that it 
by no means follows that, if the district of the Alps is a good locality for observing 
the one, it is therefore also good for observing the other. As to that I express 
no opinion. Probably a spot like Mount Whitney, where Langley observed, might 
be preferable, more particularly as the long waves are much less scattered than the 
short waves. 
2 N 
MDCCCLXXX VII. —A. 
