OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
255 
days on which there was a breeze blowing from a favourable direction, such as I have 
already indicated ; and, if the sky were partially cloudy, only those days on which the 
clouds were collected in cumuli, leaving fairly large spaces in the sky of as dark a 
blue as is obtainable in England. 
I would here interpolate that, from the nature of the loss of light by transmission 
through the atmosphere, it would be impossible to find a sky of that blue-black 
which is found at high altitudes ; the most that can be expected is a deep blue. 
Besides the ordinary meteorological observations which were taken at the Museum 
at South Kensington, I have had the advantage, through the kindness of Mr. Whipple, 
and by permission of the Meteorological Council, of obtaining complete records from 
the Kew Observatory for the days I wished to utilise. As this observatory is but a 
few miles distant from my place of observation ; and, as these records are compiled 
with every accuracy, I have utilised them for my work. 
§ VII. Atmospheric Conditions at the Riffel. 
The morning atmospheric conditions of the Riffel were perfection on each day of 
my observations, but after 2.30 P.M. they were unsatisfactory. In the mornings the 
wind was north, a quarter which is well known to Alpine men as a “ fine ” quarter, 
and the sun rose with a whiteness of surface which I suppose we can never see at 
low elevations. The sky was then intensely blue, and improved to a blue-black as the 
sun gained in altitude. The distant Oberland ranges of mountains were well defined, 
no visible haze intervening. The shadows on the distant Bietschhorn were black, and 
the snow-capped summits stood out with almost undimmed whiteness. At Thebes, in 
Egypt, where I was for three months in 1874-75, I often remarked upon the depth of 
shadows of the rocks of the Lybian range, distant some three miles from my station ; 
but the shadows in the Oberland Alps, lying some twenty miles or more away, were 
on this occasion even darker, showing that the haze caused by dust, which is always 
more or less prevalent in Egypt, was almost absent. The presumable absence of 
water particles as well as of dust on my days of observation at the Riffel made the 
atmosphere clearer than it was in Egypt even under the most favourable conditions. 
I am not stating this only from recollection, but I have records of the fact in photo¬ 
graphs which I took at the time at both places. This state of the atmosphere lasted, 
as I have said, till the afternoon, when a battle for supremacy was waged between the 
north wind and the wind coming over from the Italian side of the range. The sky 
then became more or less hazy, and the observations of sunlight were discontinued. 
The sky at mid-day, as I have said, was of a blue-black, and in fact, with a pocket 
spectroscope, the spectrum could barely be seen. On a previous occasion, when photo¬ 
graphing the sky spectrum at the same place, and, as far as can be judged, under 
precisely the same conditions, the exposure necessary to give to the plate was at 
least some seven or eight times that required in England to obtain similar results, 
