( B16 ) 



the tube. Analysing tlie ligiil of the ditrerent points of the tube 

 with a spectroscope, we find of course a dilfereiit iiiagnefic separation 

 for every point. 



We can liowever spectroscopically analyse slimdtaneoush/ the light 

 of all points of the tube. 



We have only to focus an image of the tube upon the slit of the 

 spectroscope. This spectroscope must satisfy one condition. This con- 

 dition is that to every point of the slit there corresponds one point 

 of the spectral image. In the case of a prism spectroscope, of an 

 echelon spectroscope, and of a plane grating spectroscope, this condition 

 is clearly fulfilled, but the concave grating mounted in Rowland's 

 manner forms an exception. The use of the concave grating necessitates 

 in our case the employment of the method pnijKised by Ru.nge and 

 Paschen '). 



My experiments were made in the above maimer. 



To illustrate this method I shall take the blue line of mercury (4359), 

 which divides into a sextet. 



The distribution of the magnetic force in a plane perpendicular to 

 the axis of a du Bois electromagnet with a distance of 4 mm. between 

 the poles is mapped out in a spindle-shaped magnetograra, of which 

 a part is reproduced in Fig. 1. This figure is from a negative enlarged 

 9 times. We may extinguish by means of a Nicol the light of the 

 inner components. At both sides two narrow lines remain. Fig. 2 

 is a nntural size reproduction of a magnetogram taken under the 

 specified conditions. The duplication of the outer components is lost 

 in the reproduction. The extension of the field, mapped out by this 

 magnetogram, may be better understood if I observe that 1 mm. in the 

 focal plane of the spectroscope corresponds to 1.80 mm. in the plane 

 between the poles or 1 mm. in the latter plane to 0,556 mm. of the 

 negative. Hence in Fig. 1 5 mm. corresponds to 1 mm. between the 

 poles. The complete magnetogram gives the magnetic force in a line, 

 30 mm. in length. Using a lens of shorter focus we can represent, 

 of course, a greater part of the field. In the middle of the field the 

 magnetic force is about 24,000 C. G. S. A comparison oï Held strengths 

 can be made with a decidedly higher degree of accuracy than that 

 which is given above for an absolute measurement. 



The method set forth above will be applied, of course, only in difficult 

 cases. As long as our spectroscopes of great resolving power are 

 rather cumbersome, no practical application of the method is possible. 



In many cases there will be great advantage in selecting a spectral 

 line which is tripled in the field. 



1) Kayser. Handbuch Bd. I, p. 482. 



