1906.] on Recent Progress in Magneto- Optics. 297 



cerning the details of this double refraction, have fully conlirmed 

 Voigt's theory.* 



The slides shown always refer to o?ie of the yellow sodium lines, 

 and hence the structure seen is almost entirely confined to the ex- 

 tremely small region between the components of one line. 



The line D2 splits up into three components in a moderate field. 

 Tlie theoretical course of double refraction is given in a diagram ; 

 next to it the result of observations is given. (Figs. 15 and 16.) 



On a somewhat larger scale the appearance is as now shown ; with 

 greater density the characteristic sinuous line undergoes transforma- 

 tion. 



The line D^ splits up into a quartet. Besides the concave parts 

 you will now notice a line with a point of inflexion in the theoretical 

 and in the observed curves. (Figs. 17 and 18.) 



The same phenomenon is again illustrated by the next slide, where 

 also the change which occurs with greater density is manifest. In a 

 very strong field tbe line D2 is resolved into a sextet. The inverse 

 sextet can be readily seen with the means at our disposal. But 

 the phenomena occurring between these narrow spaced components 

 could only be seen with diificalty. Only under very favourable 

 circumstances Mr. Geeso obseired the image now projected. 



All the described phenomena are qualitatively in excellent accord- 

 ance with Voigt's theory. It is certainly very interesting that the 

 theory is able to explain the complicated course of double refraction 

 by the difference between the velocities of propagation of vibrations 

 at right angles and parallel to the field. 



Magnetic Resohdion and Intensitg of Field. 



Let me again refer to our first subject, the magnetic separation 

 of the Hues. The magnitude of this separation is proportional to 

 the intensity of the field in which the source is placed. We may, 

 therefore, deduce the intensity of the field from the magnitude of tbe 

 magnetic separation. We have only to measure the distance of the 

 components of a suitable line. It is not generally known that this 

 distance can be increased with great accuracy (with an error of far 

 less than 1 per cent.). It is, therefore, far easier, if a relatively high 

 degree of accuracy is necessary, to compare the intensities of field by 

 measurements of the distance between the components than by direct 

 magnetic measurements. 



All methods used for the measurement of magnetic fields give us 

 the intensity in a point. On the other hand, the magnetic resolution 

 of spectroscopic lines can give us the intensity in all jwints belonging 



* Zeeman and Geesfc. Pi-oo. Acad, of Sciences, Amstefdam, May, 1903, 

 December, 1904. Geest. Thesis, Amsterdam, 1901, Archiv. Neerl, ser 2, T. 10 , 

 p. 291, 1905. 



