258 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
observation was about 2 atmospheres. From the first group a mean of the air¬ 
thicknesses is taken, and also the mean coefficient, and the second group is treated in 
the same way. Thus 
Air-thickness. 
k 
June 4. 
1-265 
•00190 
July 5. 
1-373 
■00211 
July 21 ... . 
1-205 
•00220 
Means . . . 
1-311 
•00207 
Air-thickness. 
k 
October 29 . 
3-404 
•00472 
November 4. 
3-428 
•00500 
November 18 . 
3-160 
■00412 
Means 
3-3.31 
•00461 
To ascertain the coefficient for one atmosphere, we use the formula 
x' — X 
- -= 
where x' and x are the coefficients for the two groups, and z and s the air-thicknesses. 
In the case before us we thus have 
that is, the formula 
becomes 
x being air-thickness in terms of one atmosphere at sea-level. 
For October 14th the air-thickness is 1‘973, and the coefficient is ‘00284 ; according 
to the above formula, the last should be ‘00249. 
Combining October 14th with the second group, we have ‘00130, which is not 
very different from that derived from Groups 1 and 2. 
Now the accuracy of reading may vary about 2 per cent, in the yellow of the 
spectrum and perhaps more in the blue, and it is believed that greater accuracy at 
af—x __ ‘00461 - -00207 
^-s — 3-331-1-311 
= ‘00126 ; 
r = Ie-* ; 
— kxK~* 
_ T -'00126A- 4 .a: 
r = Ie- 
