OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
269 
The 6-inch turbid cell was exposed for five minutes through rotating sectors having 
an aperture of 90°. 
Hence the above values have to be multiplied by 7’50 and 375 respectively, and we 
find the relative values of a unit exposure to be— 
Clear cell. 4-incli turbid. 6-inch turbid. 
1 _I_ _2_ 
10125 331 
Using the logarithmic formula, we get, taking the clear cell and the 6-in. turbid 
cell as giving points in the curve, 
H — 5832 ; 
and the value for the 4-in. cell becomes 1/10*3, which is sufficiently close to 1/10*125 
to show that it holds good. 
The optical values were also taken by means of the rotating sectors, and found to be— 
Clear cell. 4-inch turbid water. 6-inch turbid water. 
75*5 26-5 15-5 
Using the first and last as giving points lying in the curve derived from the 
logarithmic formula, we get // = *2639, and find the values to be 75'5, 26‘3, and 
15 - 5 ; a coincidence which is nearly exact. 
Now we find that 
•5832 
•2639 
2 - 21 ; 
and if we compare the value of the optical measure of the areas of the sun curves 
with the measure of the photographically derived curves in the same way in the 
results we have given, it will be found that the factor p///x is very nearly the same as 
the above. This close approximation leaves no doubt that the logarithmic formula is 
sufficiently exact to be employed. 
Another example may be quoted, which will, with a diagram, still further explain 
the mode adopted. An exposure to lamp-light was given to portions of plates 
in succession by means of the rotating disc, for 15 seconds each exposure. The 
apertures were as follows, and the densities, measured as described, are placed in 
juxtaposition :— 
Aperture. 
Relative transparency 
of negative. 
o 
8* 
177 
18* 
83 
28^ 
51* 
38i 
344 
484 
27 
58* 
22 
714 
164 
864 
124 
984 
111 
138 
u 
180 
