666 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OE 
comparison of the thermograph with the photograph, and I am inclined to believe that 
the first method gives a result which should be fairly accurate though it omits e from 
the thermograph. 
If H be taken as known, and if the fiducial line y y be correct, the limit of the 
spectrum would be beyond e; but it is hard to reconcile this with the fact that the 
centre of the last red image of the sun seen through cobalt glass would be near C, 
otherwise the absorptions shown in the photograph would agree with the breaks of 
continuity in the thermograph. 
Theoretical deductions. 
It has seemed advisable to keep any theoretical deductions to the concluding portion 
of tins paper, as the first portion contains wholly facts on which no controversy, I can 
imagine, can arise ; at the same time, it would be wanting in candour did I not point 
out what seems to me to be some evident conclusions which can be drawn from the 
experiments that I have made over such a long period. 
I have pointed out that the form of silver bromide which is sensitive to the red and 
ultra-red of the spectrum transmits the least refrangible rays to a marked extent, but 
it does not do so entirely ; in fact, we may say it absorbs all rays, the less refrangible 
the best, the more refrangible much less, and the green rays least of all. A very 
instructive experiment to repeat is to photograph the spectrum of burning coal-gas 
with this salt. . It will be found that the curve of intensity constructed as already 
indicated has the following appearance in the prismatic spectrum. 
It shows two well-defined maxima, which are situated somewhere about wave 
lengths 3800 and 7600. It may be merely fortuitous that these occupy the positions 
they do in the spectrum ; but if we are to look at the occurrence of the maxima at 
intervals, one of which has double the wave length of the other, we cannot but be 
struck with the idea that a molecule of the silver bromide is responding to harmonic 
vibrations. In the state in which the silver bromide has only one maximum at about 
wave length 3800 it seems probable that the molecule exists of only half the weight. 
The heavier molecule may be well supposed to take up those vibrations which we may 
say are an octave higher, whilst it would not at all follow that the fighter molecule 
would respond to the vibrations of the lower octave. 
