( 339 ) 
Physics. — Dr, L. H. Srertsema: “The dispersion of the magnetic 
rotation of the plane of polarisation in negatively rotating 
salt-solutions. Il. Further measurements with potassium ferri- 
cyanide.” Communication N°. 76 from the Physical Laboratory 
at Leiden. (Communicated by Prof. H. KAMERLINGH ONNES.) 
In the Lorentz jubilee volume!) I have discussed some measu- 
rements on the negative rotation of solutions of potassium ferri- 
cyanide, where the rotation of the salt was found by letting the 
pencil of light traverse the solution and water alternately and by 
measuring each time the position of the dark band in the spectrum. 
These measurements have now been continued with the same 
salt, in the first place to investigate the influence of some small 
improvements made in the apparatus and also in order to investigate 
whether the rotation constants, as they have been calculated for 
the different wave-lengths are really independent of the concentra- 
tion of the solution. 
The chief alteration in the apparatus is that now the two adjoin- 
ing tubes, one for the solution and one for water are closed by 
means of the same glass plates. While formerly, when the two tubes 
were interchanged the spectrum was considerably displaced, because 
the glass plates were not perfectly parallel to each other, this 
displacement has now been reduced to a small quantity, and hence 
more certainty has been obtained in the determination of the wave- 
length. Moreover I could make use of sunlight which enabled me 
to determine the wave length still more accurately. 
For the rest the observations were made exactly in the same way 
as before, with solutions of 1, 4 and 2 percent. For the calculation 
I have availed myself of the measurements of the magnetic rotation 
in water, published in the Bosscra Jubilee volume ?). 
The results are given in the following table. The symbols used 
are the same as in the previous Communication *) viz.: 
4, the wave length for the dark band in the solution. 
hg ” ” ” ” ” ” ” ” water. 
pa, the rotation constant of the solution 
Poa, n £ A » water for the wave-length A, 
Pans 5 5 „ the salt 
") Arch. Néerl. (2) 5 p. 447; Comm. Phys. Lab. Leiden N° 62. 
2) Arch. Néerl. (2) 6 p. 825; Comm. Phys. Lab, Leiden N° 73. 
Sar „ (2) 5 p. 457; y Pi Pi / N° 62 p. 451. 
