144 
MR. W. GEOFFREY DUFFIELD ON THE 
o 
Thousandths of an Angstrom unit. 
Ratios. 
Set A. 
Set B. 
Set A. 
Set B. 
Group I., unreversed . . . 
Group II. 
1-9 
1-5 
1-0 
1-0 
4-0 
3-3 
2-1 
2-2 
Group III., unreversed . . . 
7-8 
6-8 
4 • 1 
4-5 
In the last two columns are given the ratios of the displacements of Groups II. and 
III. (unreversed lines) to that of Group I. (unreversed lines). The ratios are not very 
different from 1:2:4 for both sets, which, however, are not in this respect in 
accurate agreement, but this may perhaps be accounted for by some fogging of the 
Diagram YI. 
plates of Set A, which renders the intensity curves rather too flat-topped and 
explains a tendency for the settings on the lines to be in them rather nearer the red 
end of the spectrum than in the photographs of Set B, in which the background is 
clear. On the side of the intensity curves on which the slope is gradual, the 
additional general exposure makes the percentage change in intensity less than it is 
