780 - 
ranged from | to 18 minutes. Parts of the mercury-spectrum were photographed, 
and the cameratube was usually so adjusted by means of the divided circle of 
the spectrometer that the mercuryline to be photographed appeared in the middle 
of the plate. Immediately after the development of the plates it was examined in 
which photo the intensities of the halves were equal for a special line. This 
could be properly observed only when the illumination of the half-shadow slit was 
uniform, and for this reason the mercurylamp had to be kept burning in a ver- 
tical position. 
On account of absorption photographs were taken 
for oxygen up to and including 2654 A.U. 
„ hydrogen RT s 2378 5 
‚… “Carbon diomide sn ¥, 2482 
For ultraviolet absorption by oxygen | find it stated!) that LiverNG and Dewar 
found absorption from 2745 A.U. upwards in a tube 165 em. long at a pressure 
of 85 atm. and from 8360 A.U. upwards in a tube 18 m. long at a pressure of 
SO atm. In my experiments the oxygencolumn of 230 cm. at a pressure of 80 
kg/em? just let the line 2805 A.U. through and no more, while for a pressure ot 
40 kg/cm? the limit of absorption was 2654 A.U. 
6. Measurements of the rotatory dispersion were commenced with: 
Oxygen. 
Before the experimental tube was filled and closed, the nicols were 
set at an angle previously calculated. The cameratube was then 
replaced by a telescope in order to ascertain what current strength 
gave equal intensities of the halves of the green or the violet mercury 
line. By a slight torsion of the experimental tube equal intensity 
was usually obtained for the blue-violet line 4858, with a current 
of abeut 35 amp, a gas pressure of 85 kg/em? and an angle of 
92° between the nicols. The necessary current strengths for the ultra- 
violet lines could then be calculated roughly by extrapolation from 
the dispersionformulae given by SiertsEMA. If / is the value of the 
current thus calculated, photographs were usually taken with currents 
of from (/— 2) and to (/ + 2) amp. and a series of careful exposures 
were then made at intervals of 1/,, or */,, /amp. between the values 
of / given by the first photographs. A current of 1 amp. gave a 
galvanometerdetlection of 1 em. so that with the currents used a 
change of 3 to 8 mm. in the galvanometerdeflection could just be 
distinguished on the negative by an appreciable difference in darkness. 
As the original negatives were too weak for reproduction, I prefer 
to give a drawing of a series of 7 photographs of a portion of the 
mercuryspectrum (4047—2755 A.U.) with hydrogen at 19°,5 and 
1) Kayser, Handbuch. Band III. p. 357. 
