778 
To guard against any displacement of the coils, the high pressure 
taps which were mounted before on the base of the coils, were now 
placed on a separate table. The temperature of the gas was regulated 
by that of a water jacket between the coils and the experimental 
tube. (These details are easily distinguishable in the accompanying 
photograph of the work room). 
5. The following remarks may be made concerning the optical 
part of the investigation : 
The collimator placed in front of the mercurylamp contained a single laevoro- 
tatory quartz lens, of 36 em. focal length for the yellow mercurylight, and 3,6 cm. 
diameter. The breadth of the slit was 1 mm, and its length was reduced by means 
of a brass plate to 2 mm. so as to prevent troublesome reflection from the inner 
wall of the experimental tube. A circular opening of 1 mm. diameter was substi- 
tuted later for this slit. 
The quartz covers were 11 mm. thick, and their diameters were 26 and 22 mm. 
respectively, 
The half-shadow (aperture 12 mm. by 12 mm.) with air separation, according to 
Gian, had a half-shadow angle of about 2°. It was fixed in a brass mount, and 
this fitted closely into a brass cover, whose ribbed sides were soldered in a cylin- 
drical tube. The gas had free access to the nicols through the openings, thus 
removing any possibility of displacement. To enable one to set the separating line 
of the half-shadow horizontal, the cylindrical tube was arranged so as to rotate in 
a ring attached to the endpiece. A tube, with a slit 11 mm. long and !/, mm. 
wide, perpendicular to the separating line, was attached to the half-shadow half 
of the nicol. : 
A GrAN-nicol, aperture 20 mm. by 20 mm. was used as an analyser; it was 
mounted in the same way so as to rotate in the larger endpiece of the experi- 
mental tube. The mount was provided with a circular scale of 180 subdivisions 
which was used in setting the nicols at a special angle to cach other. For the 
absolute measurements the construction of the nicols was somewhat modified as 
the water entered the airspace, and the nicols no longer polarised. A brass window 
(2/. mm. thick) was cemented between the halves of each nicol, and in this way 
the space between the nicols was protected on all sides by a layer of cement from 
ingression of gas or liquid. 
As spectrograph was used a Société Genèvoise spectrometer with a Cornu 
quartz prism. The telescope tube contained a single quartz lens, of 36 cm. focal 
length for yellow mercurylight, and of 3.6 cm. diameter. The eye-piece was 
replaced by a camera, which was constructed in the workshop of the Delft laboratory 
by the master instrumentmaker Mr. P. vAN DEN AKKER; the accompanying 
diagram shows the arrangement drawn to a scale of one half: 
B is a horizontal arm attached to the telescope stand; C is a fixed semi-cylin- 
drical brass drum, and D a similar semi-cylinder rotating about an axis coincident 
with the vertical central line of the photographic plate, by means of which it 
is possible to set the plate at an angle (usually 50°) to the axis of the camera 
