OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
257 
with the observations at these two dates, showed that the observed values could be 
accounted for by the theory that the loss at the station of lower altitude was due 
solely to the suspension of fine particles in the atmosphere. Other days’ observations 
in England were compared in the same way, and in very nearly every case the above 
law held good, hx, of course, varying as the zenith distance of the sun on the different 
days varied. A comparison was then made inter se of the different days’ observa¬ 
tions, and the above law, as it should, still held good. Now, on the different days 
in England on which the observations were made, the mean time of observation 
was known, and consequently the air-thickness. The air-thicknesses were reduced 
to a uniform barometric pressure of thirty inches of mercury for one atmosphere. 
§ IX. Conditions of Comparison between the Riffel and South Kensington. 
Evidently the air-thickness at mid-day on the 15th September at the Riffel could 
not be compared with the air-thicknesses at sea-level, as it by no means followed that 
the atmospheric conditions were the same, i.e., that the scattering particles assumed by 
the above theory were as abundant at an altitude of 8,000 feet or more as in London 
at sea-level in a unit volume. It should be remarked that this does not affect the 
value of the comparison of the observations made between the two places, since the 
law would equally apply, were the particles per unit volume more or less. The 
reduction of the observations showed that the particles were fewer in number, which 
is equivalent to observing sunlight through an atmosphere less than unity at sea-level, 
though the absolute air-thickness was slightly greater than unity. It is manifest 
that there is very great utility in making the illuminating value of the spectrum at 
the Riffel the standard with which to compare the values obtained with greater air¬ 
thicknesses at sea-level; since the differences in the proportions of the different 
transmitted rays are very much accentuated, and any slight errors in observation, 
which would mask the results when the differences between the observed air¬ 
thicknesses are small, are eliminated. 
§ X. Minimum Loss of Light. 
For the purpose of obtaining the minimum loss of light, I have selected observations 
made on seven days, all of which are compared with the Riffel observations. The 
atmospheric conditions were favourable, as will be seen by the meteorological Tables 
annexed. These days are divided into two groups of three, leaving one odd day. 
The first three days are June 4th, July 5th, and July 21st, when the observations 
were made with a thickness of about 1*3 atmosphere. The second group comprises 
observations 29th October, 4th November, and 18th November, when equally satis¬ 
factory atmospheric conditions existed, with thickness of about 3*3 atmospheres. 
The seventh day is 14th October, when the thickness of atmosphere at the time of 
MDCCCLXXXVII.—A. 2 L 
