J 02 



there was a watejjacket, tlirougli which a nlreaiii of water could 

 be led. The projecting eiids of Ihe test lube are enclosed in cotton 

 wool to ensure a unitbrui teni|)erature. The strength of current 

 amounted to about 2 ampères, and was read down to 0,002 ampère 

 on a carefully ganged ampère metre. Of the optical arrangement we 

 should mention the source of light (quartz amalgam lamp of Hkrakus, 

 or arclamp), from which the beam of light passed through a penumbra 

 prism according to Jellet, with a penumbra angle of 2°, a colli- 

 mator, the slit of which was immediately behind the penumbra 

 nieol, the test tube, a nicol with 12 x 13 mm. aperture as analyzer, 

 the rotation of which could be read in minutes on a graduated 

 circle, and the prism with eye-piece of a Hilgkr spectroscope with 

 constant deviation. 



The line of demarcation of the penumbra nicol is placed hori- 

 zontally, so that when the arclamp is used we observe a spectrum 

 in the eye-glass consisting of two parts lying above each other and 

 divided by a sharp line. Through rotation of the prism different 

 parts of the spectrum can successively be brought in the middle of 

 the tield of vision. If the current in the coil is closed, a black band 

 is observed in both parts of the spectrum. These two bands, how- 

 ever, are slightly displaced with res[»ect to each other. Halfway 

 between them a place can always i>e indicated where the intensity 

 of the light is the same in the two parts. At this place the adjust- 

 ment was brought about by means of rotation of the analyzer causing 

 a displacement of both bands. For this purpose the eye-piece was 

 provided with a wide ocular slit. After reversal of the current this 

 adjustment was repeated; the angle over which the analyzer has 

 been turned, is double the angle over which the plane of polarisation 

 in the titanium chloride has been turned. In observations with the 

 (]uartz amalgam-lamp tije collimator slit was taken pretty wide, which 

 caused a great many slit images to be seen in the reading glass, each 

 divided into two parts by a horizontal line. .Just as above the adjust- 

 ment can then be made at equal luminous intensity of the two parts. 



From readings of thermometers in the supply and the leading 

 off of the water in the jacket the temperature of the test tube can 

 be derived. 



The first series of measurements have been carried out with the 

 quartz amalgam lamp. The slit images used for this are those cor- 

 responding to the lines : 



Hg 5780, the two mercury lines lying close together. The images 

 of the two lines overlap for the greater part; the readings are 

 reckoned to correspond to the mean of the two wavelengths. 



