( 1090 ) 
method first applied to similar researches by Peccianti'), then by 
GrisLer ®). As our present equipment bears a temporary character, 
and we expect to dispose of better appliances at a later date, the 
following short description of the apparatus used may suffice. 
The light coming from an are lamp of 25 A. passes through a 
JAMin interferential refractor, by which it is split up into two beams, 
29 millimeters apart. On the paih of one of the beams there was 
a glass tube, 12 centimeters in length, into which a controlable quan- 
tity of bromine vapour or of nitrogen peroxide could be introduced, 
while the other beam traversed two pieces of plate glass, identical 
with the ones shutting the tube. With both gases observations were 
made at the temperature of the room. Iodine vapour, on the con- 
trary, was examined at 53° C. For that purpose two equal glass 
tubes, 40 ¢.m. long, were placed one in each beam, a Heraxus electric 
furnace enveloping them both, so that their middle parts, for a length 
of 23 ¢m., could be evenly heated to the same temperature. One 
of the tubes contained some iodine. They both communicated with 
the open air by narrow side-tubes. 
By means of lenses the horizontal interference-fringes were focussed 
on the slit of the spectrograph. As a low dispersion apparatus we 
used a HiLerr constant deviation spectrograph, with one dense flint 
prism; a few observations were made with high dispersion apparatus, 
consisting of a Row .anp plane grating (working surface 8 5 ¢.m., 
5680 lines to the e¢.m.), two silverel glass mirrors of 150 and 
250 e.m. foeal distance respectively, and camera. 
The annexed plate shows threefold enlargements of some of the 
spectrograms. Much detail is lost in the reproduction. When there 
was no selectively absorbing substance present in one of the beams, 
the interference fringes were perfectly smooth and almost horizontal, 
apart from their fanlike spreading with increasing wavelength. As 
soon as the absorbing gas is introduced, the velocity of certain waves 
has inereased on the one path, of other waves diminished; so the 
fringes suffer local displacements, quickly increasing as we approach 
an absorption line, and thus show very intricate incurvations. In 
our arrangement a downward incurvation of the fringes means 
increasing velocity of the corresponding waves in the vapour, and 
therefore, decreasing index of refraction of the latter; an wpward 
incurvation of course indicates the reverse. 
1) Puceranti, Mem. Spettr. Ital. 83, 133, 1904; Nuovo Cimenio, Ser. V, Vol. 
IX, 303 (1905). 
2) H. Geister, Zur anomalen Dispersion des Lichtes in Metalldämpfen, Diss., 
Leipzig, Barth, 1909. 
