( 1091 ) 
The first and second spectrum give the anomalous dispersion and 
the absorption in zodine vapour. Proceeding in the direction of in- 
creasing wave-leneths towards the sharp edge of any absorption band, 
we see the fringes curve steeply down, which proves that the index 
of refraction is quickly diminishing; within the absorption band the 
index appears to increase, rather quickly at first, then slowly *), until 
in approaching the next band it again falls off steeper and steeper. 
This process repeats itself at every fluting, without exception. 
The resolving power of the Hireer spectrograph was not sufficient 
(9 permit of distinguishing the separate lines composing the flutings 
of the iodine spectrum; but by analogy with quite conspicuous pheno- 
mena observed in the case of nitrogen peroxide (as will soon appear), 
it can hardly be doubted that also in the iodine spectrum each line 
of a fluting causes the index of refraction to sink on its violet side, 
to rise on its red side, and that the apparently continuous increase 
of the index within each fluting results from the joint action of the 
anomalies, due to the separate lines of that fluting. This inter- 
pretation — which is in keeping with the dispersion theory — is 
strongly supported by the results of our observations on nitrogen 
peroxide. 
With bromine vapour we did not succeed in photographing equally 
sharp and distinet anomalies of the dispersion as those obtained with 
iodine. This may perhaps be dne to the fact that, if one compares 
such quantities of both vapours as will transmit nearly equal fractions 
of the ineident light, the intensity varies less within the bromine 
flutings than within the iodine flutings. With equal average absorption 
there are stronger contrasts in the spectrum of iodine, than in that 
of bromine vapour. Nevertheless, on examining the third and fourth 
spectrum, we may safely conclude that the anomalous dispersion in 
bromine vapour bears entirely the same character as that in iodine 
vapour. 
The next three. spectra. relate. to. -mitrogen; perdxride; + the 
absorption spectrum (photographed while one of the interfering beams 
was screened off) is placed between two spectra showing the anom- 
alous dispersion; number 5 was obtained with gas of lower, 
number 7 with gas of higher density. Exposure and development 
were so timed, that on the one photograph the region between 2 4400 
and 2 5200, on the other that between 25200 and 26200 comes 
out to advantage. Among the hundreds of lines visible in the NO,- 
spectrum we could not find a single exception to the rule, that every 
1) Cf. H. Getster, l.c. p. 24. 
