( 475 ) 
The thickness of the air-layer in this étalon was nearly 25 m.m. 
With this distance and using the light of the mereury line 5790 
in the magnetic field the limit of the method is being rapidly approached. 
Hence the accuracy of the results obtained with the 25 m.m. étalon 
is in our case hardly superior to that to be reached with the 5 m.m. 
apparatus. 
The arrangement of the apparatus was described with sufficient 
detail on a former occasion *). For the purpose now in view it was 
desirable to investigate exclusively the vibrations parallel to the 
magnetic force. A calespar-rhomb therefore was placed between the 
source of light and the first lens. Two images of the radiating 
vacuum-tube are now obtained near together on the étalon, the 
non-desired one being screened off. A photograph was taken with 
the field on, and before and afterwards one with the field off. 
Besides the inner ring, always also the second ring, in some cases 
also the third and fourth one, was measured and the result used in 
the wave-length calculation. 
The formula for the calculation is the one first given by Fasry 
and Perrot, still remarkably simplified in our case ’). 
In the following table the results are given relating to the mercury 
line 5791. The first column contains the number of the experiment, 
the second one the reference-number of the spectrogram; A2, is the 
change of wavelength of the central component. The field intensities 
are given in the last column. Their relative values, which are only 
necessary for establishing the law connecting displacement and strength 
of field, are exact. These numbers must be increased with 1 or 2°/, 
in order to reduce them to gausses. 
Experiment Plate n°. AP in Ase. H. 
1 208¢ 0.0085 12700 
2 2095 0.0088 12700 
3 211 0.0169 20700 
4 212¢ 0.0074 13950 
5 214° 0.0201 20600 
6 218% 0.0367 28250 
7 218¢ 0.0358 28250 
8 219% 0.0360 28250 
9 220% 0.0353 29170 
10 2204 0.0406 29780 
1) Zeeman. These Procce dings December 1907. 
*) See These Proceedings December 1907, February 1908. 
