OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
261 
§ XII. Loss of Light other than by Scattering. 
The question arises if there is a loss of light from any other source than the 
scattering by small particles, and an answer is at once furnished by every-day 
observation. We know that it is ; that a hazy day diminishes sunlight, though often 
without materially altering the ratio of the components of the light. Now light, 
qua light, I did not measure on all these occasions; but here the value of the 
measures taken by means of the candle comes in, assuming that the candle is invariable 
or only varies slightly. Mr. Vernon Harcourt tested a candle such as we employ, 
and got from it a result more satisfactory than with a standard candle; and, as the 
same brand of candle is always employed, it is believed that the same measurements 
hold good. 
With the instrument unchanged in any respect, the slits having the same width in 
both cases, an estimation of the total illuminating value of sunlight can be at once 
ascertained. It happens that on two days which I have selected, viz., 4th June and 
29th October, what I may call the candle-value of the spectrum was ascertained. 
Now the illuminating value of the spectrum, taken by means of the candle, as already 
has been said, is not such a satisfactory method as that I have latterly employed, and 
on which the foregoing measures have been based ; but an average value for k can 
be readily ascertained. On the 4th June, about a quarter of an hour later than the 
time when the printed observations were made, the value of the ray at 47 on my 
scale was 80, and for the 29th October, also about a quarter of an hour later, it was 
62, or 1 to '7 75. The value for this ray at the adopted times of observation on the 
above days is 114 - 3 and 89‘0 respectively, or 1 to - 778. This shows that on these 
two days any loss of light, except that due to scattering, was very small. On 
some other days, however, I adopted a different plan, which I had carried out, not 
for the purpose of applying them to these results, but for estimating the photo¬ 
graphic values of skylight and sunlight. Fortunately, also, these measures were 
carried on at the Eiffel: observations of no small value, as it proves. 
§ XIII. Loss of Total Light by Transmission through the Atmosphere. 
Before entering into this more fully, something must be said as to the value of total 
sunlight as derived from my observations, assuming for the present that there is no 
diminution of light except from the scattering by the small particles. Having 
obtained the coefficient of transmission per atmosphere for each ray, it is easy to 
construct curves representing the luminosity for every air-thickness, and, having so 
constructed them, to find the value of the areas of each. These areas then represent 
the values of total illumination which would be observed, were the total light under 
measurement, as General Nesting and myself have shown in the paper before quoted. 
Having found the areas, it is easy to see if any law holds good connecting total light 
