366 Messrs. du Bois and Rubens on Refraction 



In the first place Dr. Lobach* and one of usf published 

 data for the dispersion of Kundt's and Kerr's magneto-optic 

 phenomenon respectively in these three substances ; those 

 numbers refer to the same colours we have now used again. 

 We so hope to contribute towards the experimental deter- 

 mination of strictly comparable optical constants. 



Secondly, it is comparatively easy to obtain these metals 

 electrolytically in wedge-shaped films. We may therefore 

 safely assume that our metal prisms left little to be desired. 



Lastly, the considerable amount of their refraction con- 

 tributes towards the accuracy of measurement. In addition 

 we shall occasionally have to discuss the behaviour of certain 

 metals with much smaller refractive indices ; however, w r e 

 have not made any experiments on these (see § 16). 



§ 3. The experiments were carried out almost in the same 

 manner and with the same spectrometer as those made by 

 Prof. Kundt in Berlin (B. pp, 1389-91). We did not use the 

 heating arrangement there described ; certain other special 

 appliances were, however, devised, to be described in due time. 

 Readings had previously either been taken off the spectro- 

 meter's divided circle or with a micrometer-eyepiece, Neither 

 of these plans was adhered to in the present case ; the adjust- 

 ments being made instead by turning the micrometer-screw 

 of the spectrometer's telescope (magnifying Kkfold)* This 

 having been done by one of the observers, the reading on the 

 divided head of the micrometer-screw was taken by the other, 

 observing through an auxiliary telescope placed at a distance. 

 One division on the divided head corresponded to a rotation 

 of 4 /r, 20 of the telescope round the spectrometer's vertical axis. 



Refracting angles and deviations are always given in 

 seconds, each being obtained from 20 readings. Generally 

 the collimation was checked before each such set by observa- 

 tions through the lateral windows, according to the rules laid 

 down by Prof. Kundt (B. p. 1390). 



§ 4. Checks have been applied to the method in considerable 

 number by Prof. Kundt (A. pp. 263-265). We have there- 

 fore only had to add a few measurements in order to test the 

 method of collimation. A perfectly plane and parallel glass 

 proof-plate of Steinheil's was blackened in such manner as to 

 leave open four windows, of breadth *2 centim. and height 1*0 



* Lobach, Inaugural dissert. Berlin 1890 ; reprinted Wied. Ann. xxxix. 

 p. 358 (1890). 



t du Bois, Wied. Ann. xxxix. p. 38 (1890); Phil. Mag. [5] xxix. 

 p. 264 (1890). 



