Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/23

Rh be wondered at. For it must be recollected that the absorption by aqueous vapour takes place principally in the infra-red region of the spectrum, and, if it were entirely confined to that region, the presence or absence of the aqueous vapour would have no practical effect on the luminosity of the visible rays transmitted. The absorption by this vapour where it takes place is large and follows the ordinary laws, as already stated, being principally in the infra-red of the spectrum, the heating effect will therefore diminish much more rapidly than the total illuminating value of sunlight as increased thickness of atmosphere is penetrated. In other words, the amount of light transmitted from the sun bears no comparison with that of its total heating effect.

It should be remarked that the portion of the spectrum used in these observations is almost free from any absorption by aqueous vapour, and, consequently, the scattering effect of the small particles in the atmosphere is probably almost entirely the cause of the loss of light from it, and enables a factor for such scattering to be arrived at without much difficulty. The measurement of the amount of light transmitted, through different thicknesses of atmosphere has hitherto been almost entirely made on star and moon light, and, for reasons given in Part I. of this paper, the coefficients arrived at are probably slightly too large for the transmission of sunlight.

The results of these observations made at different altitudes, combined with others made in the laboratory, point to a probability that the particles which selectively scatter light are due to water. Their dimensions are probably very closely, if not exactly, the same, and the "mist" particles are large compared with them.