( 1089 ) 
it has been treated of on several occasions already *), and will also 
in the future continue to form a subject of close investigation. 
As to the first question, we notice that the dispersion theory has 
settled it to a certain extent. According to tbat theory there is a 
necessary correlation between selective absorption and the rapid 
variation of the index of refraction for waves differing little in length 
from the absorbed waves. The whole of experimental evidence sup- 
porting the dispersion theory in general, may thus be considered 
to plead in favour of the thesis, that really every absorption line 
involves anomalous dispersion of neighbouring waves. The hypothesis 
that many solar phenomena are perhaps produced by anomalous 
dispersion, could therefore not be deemed unfounded or premature 
even in 1900, when introduced by one of us*), although at that 
time the peculiar course of the index of refraction near narrow 
absorption lines, had really been observed with very few metallic 
vapours only. Direct observations have considerably accumulated since 
then. From researches by LuMMER, PRINGsHEIM, Woop, Epert, SCHÖN, 
Puccranti, GeisLeR, LADENBURG, and others, we know that hundreds 
of lines determine variations in the velocity with which neighbouring 
kinds of light are propagated, exactly in the manner as required by 
the dispersion theory. 
The degree in which the phenomenon showed itself was very 
different for different lines, and entirely dependent, of course, on 
the conditions of the experiment. With innumerable lines it has 
not yet been observed at all. But in view of the well-established 
dispersion theory the supposition that certain absorption lines or 
absorption bands give no anomalous dispersion ®), is a more hazardous 
one, than the supposition that the phenomenon will manifest itself as 
soon as we shall have realized the proper conditions. 
We propose to investigate those conditions for a number of gases 
and vapours, and to inquire whether selective absorption really is 
always accompanied by anomalous dispersion, as the theory demands, 
or whether there are exceptions, which would make a correction of 
the theory unavoidable. 
The observations to be described briefly in this paper bear upon 
iodine vapour, bromine vapour, and nitrogen peroxide. We used the 
1) Proc. Roy. Acad. Amsterdam, XII, 266 and 466 (1909), XIII, 2 and 881 
(1910/11). Les raies de Fraunhofer et ta dispersion anomale de la lumière. Le 
Radium VII, Oct. 1910. 
2) Proc, Roy. Acad. Amsterdam, IL, 575, (1900). 
3) Cf. e.g. Hare and Apams, Astroph. Journ. 30, 230, 1909, 
