OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
271 
3-in. turbid ; 3 min. exposure. 
Aperture. 
Density. 
Equivalent on 
above scale. 
Therefore 1 = ‘984. 
Reduced to 1 min. exposure, 
1 - -328. 
m 
284 
58i 
864 
84 
51 
22 * 
13* 
18 
28 
59 
84 
191 
189 
Using the first and last for points in the logarithmic curves, we get the observed 
values 
2-595, -795, and ‘328 ; 
calculated 2‘595, ’786, and ‘328 ; 
/x = -677. 
The optical values were next observed, from which /i'='306 when gjf = 2'21, 
which agrees with the foregoing example. 
§ XVIII. The Measurement of the Photographic and Optical Values of Total Intensity 
equivalent to the Measurement of a Single Ray. 
A remarkable deduction now presents itself from the fact that, if we divide g and g 
by k in the results given by plotting the areas, we find that the results are numbers 
which are about 105 and 235, and these represent wave-lengths 5570 and 4540 
respectively; so that, if we observe the total value of light optically, it is 
equivalent to observing monochromatic light of X 5570, and if we use bromoiodicle of 
silver for registering the intensity it is equivalent to measuring a ray of X 4540. 
We may apply any of the results obtained by astronomers to find the value of k. 
Coefficient 
of atmospheric 
transmission. 
k 
Langley (on Etna). 
•880 
1-274 
•00122 
* Pritchard (at Cairo). 
•843 
1-704 
■00164 
,, (at Oxford) .... 
791 
2-333 
•00224 
Bouguer . 
•812 
2-070 
•00199 
Seidel and Pickering .... 
■794 
2-311 
•00222 
MiiLLER. 
•825 
1-924 
•00185 
With these factors we may construct the curves of illumination for any air-thickness. 
* Professor Pritchard’s maximum value, as far as I have calculated it, is very close to mine, viz., 860. 
