775 
of equal intensity of illumination is displaced along the spectrum. 
When using a discontinuous spectrum (iron arc, or Herarus quartz 
mercury arc) equal intensity has to be obtained between the halves 
for a definite line of the spectrum. From a series of photographs 
for different positions of the polariser Lanpavu obtained that particular 
position in which there was equal illumination for a special wave 
length; the current was then reversed and the series of photographs 
repeated. Such a method of operating is tedious, but it has the 
great advantage of giving the required angle of rotation direct from 
the photographs without the measurements, which are required for 
determining wavelengths in a continuous spectrum. 
3. In order to adapt Lanpav’s method to the investigation of gases, 
the following modifications of SteRTSEMA’s apparatus had to be made: 
a. The polarising nicol had to be replaced by a half-shadow nicol, to which the 
slit was attached. When the halfshadow was used as analyser, the great length 
(about 230 cm) of the high pressure tube brought the slit too far from the 
source, and the light was then too feeble. 
b. Equal intensities had to be obtained by varying the current, as the half- 
shadow nicol atlached to the rotating end of the experimental tube had to be 
maintained in a fixed position. 
It was now possible, by slightly varying the current for successive 
photographs, to determine accurately, for different lines of the spectrum, 
the particular current, at which equal intensities were obtained. These 
currents are inversely proportional to the rotatory constants, and the 
constants can be expressed in terms of a standard line as unit. 
A preliminary investigation was made to see if it was not possible 
to arrange the nicols outside the experimental tube. As the quartz 
covers were ordered of equal but opposite rotations, the measurements 
would have been simplified by attaching the analyser with a divided 
circle to the spectroscope, for I should then have been able to read 
the rotations directly. With the quartz plates placed between the 
nicols, however, perfect extinction could not be obtained, so that 
for the determination of Verrprr’s constant in absolute measure I 
was obliged to have recourse to a comparison with water, for which 
the constants have been determined by Stertsema') and Lanpav. It 
was of advantage then that there was nothing but gas between the 
nicols, thus eliminating the influence of repeated reflections and of 
the magnetic rotation in the quartz plates. 
4. A diagram of the apparatus is given in Fig. 1. 
L is a Heraeus quartz mercury lamp, A a collimator, C and D the coils, FF 
1) SIERTSEMA. Arch. Néerl. (2) 6. p. 825. 1901. 
