18 
CAPTAIN W. DE W. ABNEY ON THE TBANSMISSION 
larlj over the latter, for in it the water absorptions play such an important role that the 
law of scattering could hardly be taken into account also. It will be noticed that obser¬ 
vations have been made at all times of the year, and that the winter observations, when 
the surface of the ground at Grindelwald has been covered with snow and the air has 
been intensely cold (the thermometer sometimes showing as much as 40° F. of frost), 
give the coefficients of least absorption. The writer has held that most of the 
scattering particles are water in an extremely fine state of division, and that there is 
a probability that such is the case is shown by the fact that the most transparent 
atmosphere is that in which there is most warm aqueous vapour present. A change 
in the summer to what artists call an “ atmospheric landscape ” invariably shows a 
higher coefficient of absorption. Whatever these particles may be, there is no doubt 
that they are in the same volume fewer in number at high altitudes than they are 
at lower ones ; as the coefficient of absorption is so much less in the former case than 
in the latter. Any one who has observed the sky at these higher altitudes will have 
observed that when the sun is brightest and highest, the sky is blackest, that is, 
that there are fewer scattering particles. 
XXXIIL —of Light. 
It is somewhat difficult to know as to what standard of light to refer the light of 
the sun. The Author has made many comparisons between it and an amyl acetate 
lamp, and has come to the conclusion that at midday at sea level, and in the clearest 
atmosphere, the brightness of the sun exactly overhead would be close upon 7000 such 
standard lights at 1 foot distant from the screen, which would be equivalent to 5600 
standard candles at the same distance. For the purpose of this paper, it is, however, 
unnecessary to refer the light to any particular standard, since all that was sought for 
was to obtain a comparison of the losses suffered on any one day by the light after 
passing through various thicknesses of atmosphere. It wdll be noticed in the various 
tables that the calculated brightness at the zenith varies very considerably even at the 
same altitudes. This must be laid down to one of three causes : (l) either an error 
in calculating the coefficients ; (2) a slight haze of coarse particles intervening; or 
(3) owing to a varying sensitiveness in the paper used. Where the actual observed 
intensities throughout a day do not vary much more than those calculated, it may be 
presumed that the coefficient is not very far from the truth, and, consequently, the 
variation will in all probability be due to the second cause. As stated before, there 
was a change made in the paper which occurred in 1889, and it wull be found that 
the intensities are practically the same before and after the above year. 
XXXIV.— Descri'ption, Tabulation, and Discussion of Residts. 
The results which are tabulated do not show by any means all the observations 
made. They have been carried on for four years, and only those days are recorded 
in this paper when the sky has been practically cloudless whilst the observations have 
