( 575 ) 



dispersion and the distance oC adjacent orders of tiie same line. 



Let / denote the Nvavelen!j,lh of the lijj,hl considered, ji Ueing' the 

 index of refraction, 6 the angle of dittVaclion, t the thickness of the 

 glassplates, .v the width of the steps of the echelon. 



Then in the case of normal transmission : 



dd t { d\x\ 



,«=.üO""^* -'•■,y (^> 



A being the distance of adjacent orders, we have fnrther: 





A — (2) 



J). 



3. The most simple hypothesis that could be made was, that of 

 the two lines under consideration only 5791, which exhibits asym- 

 metrical separation and not line 5770, would show a displacement 

 of the middle line. 



In order to test this hypothesis I used the echelon spectroscope 

 in such a manner that different lines could come sim.ultaneonsly 

 under comparison. The ordinary manner of using the echelon only 

 permits the examination of one line at the same time or at least 

 only of lines which differ by a small fraction of an Angstrom unit. 

 We may however place the steps of the echelon in a horizontal 

 position, the slit of the echelon collimator being also horizontal, thus 

 rotating these parts through 90° from the position commonly used. The 

 slit of the auxiliary spectroscope may remain vertical. This arrangement, 

 which is principally that of Newton's crossed prisms or that of 

 Gehrcke's 'Tnterferenzpunkte", has the advantage of showing simul- 

 taneously the beha\'iour of different lines. To every spectral line 

 correspond small horizontal liiies, the length of which is determined 

 by the width of the slit of the auxiliary spectroscope. It depends 



^ upon the position of the echelon whether two 



or one of the orders of a line will be visible. 

 Fig. 1 represents that part of the field of 

 view, which relates to the yellow mercury 

 lines. The lines a and b represent the successive 



^. orders of line 5770, a' the only visible order 



of line 5791, supposing that the sntall vacuum 



^ tube, charged with mercury vapour, is out of 



the field. 

 Durijig the establishment of the magnetic field the well known 

 components are seen moving upwards and downwards. Moreover 



