256 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
indicating an absence of scattered light, and a consequent increase in direct sun¬ 
light. I think, for observations on visible radiation, such a locality as I selected in the 
Alps is preferable to one of equal altitude in a warmer climate. In Professor 
Langley’s report* on his observation at Mount Whitney, he describes the atmosphere 
below him as being filled with dust, his lower observing station lying in it. Now in 
the Alps, on days such as I have described, this dust haze is absent. I have looked 
down some 10,000 feet and failed to discover it. For these observations perfect dry¬ 
ness of air is not a necessity, though undoubtedly for observations of the dark rays 
and extreme red rays it must be ; aqueous vapour, except it be present in very 
large quantities, does not affect the visible radiation from the sun at these high 
altitudes. When pointing a pocket spectroscope at a distant horizon, I have failed to 
see any of the rain-bands even when the atmosphere was notoriously damp. The 
only time at an elevation of 8,000 feet when I have seen the rain-band has been 
when rain has been freely falling. I am not asserting that the rain-bands are never 
present except under such circumstances, but only that I have not seen them. It has 
been necessary to be somewhat prolix in describing the atmospheric conditions at 
the Eiffel, as my standard solar spectrum is derived from my observations taken on 
September 15th, 1886, at noon. 
§ VIII. Law of Diminution of Light. 
The observations carried on throughout the year were undertaken more to 
obtain a meteorological record than for any other purpose, and it was not till 
November last that any comparison of the curves I had plotted was undertaken, nor 
did I opine that any set law governed the absorption of the different rays. 
Langley’s results, contained in the volume I have already quoted, rather forbade 
the idea that any exact or even approximate law prevailed, since certainly his results 
gave no clue to any. The plotting of the observations was made on squared paper, 
and through the points thus obtained a smooth curve was drawn by hand ; and it will 
be seen that all the observations lie very close indeed to the curve so drawn. From 
the curves the value of illumination at each unit of my scale was noted and tabulated. 
From such tabulation it became easy to try whether any particular law governed 
the loss of light by different rays after transmission through a thickness of air. 
Thus the intensities of visible radiation, as measured at the Eiffel on September 15th, 
could be compared with those at South Kensington on July 1st. 
A first trial of Lord Eayleigh’s theoretical law for loss of light caused by the 
scattering of small particles, I'=Ie~ * xK ~ y (where I' and I are the transmitted and 
original intensities, k a constant, x thickness, and X the wave-length of any ray), 
* ‘ Professional Papers of tlie Signal Service (U.S.A.),’ No. XV., “ Researches on Solar Heat and its 
Absorption by the Earth’s Atmosphere,” Langley. 
