OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
265 
This gives a, the coefficient of transmission, = '822. It may be imagined that 
the same result would not obtain if the coefficients of absorption for the different 
rays were other than those obtained from Lord Rayleighs law. We have at hand 
values of the atmospheric absorption for the different rays which Professor Langley 
has adopted. They are to be found in the volume to which we have referred. 
For the purpose in view, I have assumed that my estimation of the illuminating 
value for one atmosphere-with a coefficient of '001183 holds good, and constructed 
curves with the coefficients of absorption which he gives, and then taken the areas 
as giving the value of total illumination at different thicknesses. The values are, for 
0, 1, 2, 3, and 4 atmospheres— 
1,000, 657, 393, 236, 142, 
and the values obtained by using the logarithmic formula are— 
1009, 657, 394, 237, and 142, 
a sufficiently close agreement to need no comment; but it should be remarked that, as 
[A — -5109, the coefficient of transmission a = '6 nearly, a value far lower than has 
been found by astronomers. 
Finally, I give the results by the above method, which are obtained from the areas 
of the curves derived from the minimum coefficient of absorption (/r ='0013), which 
are as follows for 0, 1, 2, 3, and 4 atmospheres, viz. :— 
762, 662, 578, 504, 439. 
Taking 762 and 439 as points on the curve Ie~^ ; = T, we get the following numbers 
(fi = '1385)— 
762, 664, 578, 504, 439 ; 
which gives the coefficient of transmission = '869 ; which, it will be seen, is higher 
than any value assigned to the coefficient of atmospheric transmission that has been 
obtained by astronomers ; but to this I shall refer presently. 
§ XIV. Observations of Total Light by Colour-blind Persons. 
It was interesting to ascertain what would be the effect of the observations of 
total light by a colour-blind person as regards accuracy of result in comparison 
with a normal-eyed person. 
To ascertain this, it was supposed that the observer R # had taken the luminosity 
observations, and the curves for the sunlight w T ere reduced proportionally by the 
amount by which the normal curve of the electric light was shown to be reduced 
by this observer—an inadmissible assumption, as General Testing and myself have 
shown. The curves so reduced were plotted out, and the areas taken. 
It was found that, when Jc = '0013, for 0, 1, 2, 3, and 4 atmospheres the areas were 
532, 455, 389, 334, 287. 
* See Bakerian Lecture, 1886. 
2 M 
MDCCCLXXX VII. —A. 
