260 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
I. 
II. 
III. 
February 15 . 
•963 
■753 
March 23 . . 
•971 
■799 
•960 
■733 
•967 
■775 
•961 
■740 
•968 
■781 
March 29 . . . 
•945 
■651 
55 ... 
•9*9 
. . 
March 31 . 
•930 
■578 
April 24 . 
•975 
■826 
55 ... 
•976 
■831 
May 1 . . . . 
•961 
■740 
I. 
II. 
III. 
May 2 ... . 
•974 
■819 
May 29 
■973 
■813 
55 ... 
•976 
■831 
June 22 . . . 
•979 
■850 
September 12 
•958 
■722 
55 
•959 
■728 
September 15 . 
•978 
■844 
55 
•976 
■831 
55 
•975 
■826 
November 25 
•967 
■775 
Now tlie mean of these numbers in Column II., which are Langley’s measures, gives 
his adopted coefficient of absorption as '968 for an air-thickness, equivalent to 1 dcm. of 
mercury, and the coefficient of transmission for one atmosphere (760 mm. of mercury) 
as 781. The numbers in Column TI. are apparently very concordant, but the coefficient 
of absorption, it must be recollected, is only for a thickness of atmosphere of 1 dcm. 
of mercury, and the differences are much more apparent if we reduce them all to 
coefficients for one atmosphere, as we have done in Column III. For instance, if we 
take the highest and lowest values in Column II., which are '930 on the 31st March 
and '979 on the 22nd June, and reduce them to the coefficients for one atmosphere, 
we find that they are '850 and '578 respectively, and the mean of the series is *774. 
Similarly at page 25, Table VI., of Langley’s work the highest and lowest coefficients 
of absorption for an atmosphere equivalent to 1 dcm. of mercury are '980 and '872, 
which at one thickness of atmosphere are '858 and '351. 
Again, in the Table p. 151 for X 440, the highest and lowest coefficients for an 
atmosphere equivalent to 1 dcm. of mercury are '974 and '907, which at one 
atmosphere give coefficients of '819 and '476, whilst the lowest but one, which is '935, 
gives '601 for one atmosphere. 
These results have been brought forward to show that in the limits of visibility 
of the spectrum there are wide discrepancies in the coefficients of transmission, as 
might be expected, in what may be called uncompensated readings, and it appears that 
by adopting some method analogous to that which General Festing and myself adopted 
in our apparatus—of balancing one side of the bolometer with the spectrum and the 
other with a fraction of total sunlight—perhaps such great differences in the coefficients 
might be eliminated. It is at first sight somewhat misleading to tabulate coefficients 
of such small thickness of atmosphere, since in every case the apparent deviation from 
the mean will appear slight. 
* This figure is obliterated in my copy. 
