OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
253 
§ II. Method of observing with Sunlight. 
During the whole of these observations but one instrument was used, so that they 
compare absolutely with each other. A silvered surface was used to reflect the sun¬ 
light on to the condenser, and the image of the central portion of the sun’s disc was 
focussed on the slit and kept upon it. It became a matter of importance that the 
mirror should remain untarnished, as General Festing and myself have found that 
tarnish materially alters the proportions of the reflected rays. These results we 
intend to bring forward shortly. 
§ III. Times and Places of Observation. 
From November, 1885, to January, 1887, at my laboratory at South Kensington, on 
every possible occasion when my official duties would allow, I have taken observa¬ 
tions of the colour composition of sunlight. But as those before June 1886 were 
taken by our first plan, I have not included them here. Where my laboratory is 
situated, in South Kensington, the E.N.E., E., and S.E. are cpiarters in which the 
densest parts of commercial London are situated, and consequently, if a wind from 
any of these quarters prevails, the observations are liable to be marred by smoke ; but 
to the N.N.E., N., N.W., W., and S. there is a comparative freedom from any such 
source of error ; and with a wind blowing from those quarters the sky can be as 
blue as it is in the most uninhabited part of the country. This is important to 
remark, since it may be thought that the neighbourhood of a big city like London is 
an unsuitable one for making sunlight observations. In fact, a comparison of results 
obtained at South Kensington with those obtained in what is termed pure country 
air, shows that the latter has no perceptible advantage over the former when the 
wind is in a suitable quarter. 
In September, 1886, I took my apparatus to Switzerland, eventually bringing it to 
my old observing station on the Eiffel, Zermatt, at the height of about 8,000 feet. 
On three different days I made observations under the most favourable circumstances, 
which will be described later on. In England my observations were usually made at 
about either 10, 12, 2, or 5 o’clock; sometimes more than one set was taken on each 
day. I thought it inadvisable that anyone except myself should take readings, in 
order to avoid any readings which might be difficult of collation. 
§ IV. Length of Time required for a Set of Observations. 
It may be interesting to note the time it takes to complete a whole set of observa¬ 
tions. The way I proceeded was as follows:—The adjustment of the standard 
light was so made that in most cases the full aperture, 90°, of the rotating discs allowed 
light to pass which nearly coincided with the illuminating value of the rays of 
