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part of the flame, where the density of the sodium vapour increases 

 from left to right. In a structure of this kind, waves, for which the 

 vapour has a great index of refraction, deviate towards the right, 

 e.g. ;S: G. They arc not intercepted by Q and consequently reach the 

 slit «S,. In fact, the presence of the sodium vapour allows similar 

 waves to enter that slit even in larger quantity than they would do 

 without it, for rays of this kind, issuing from the unco\ ered half 

 of A, which if travelling in a straight line would be intercepted by 

 Q can, when refracted, penetrate the lens B. 



The case is entirely different for those kinds of rays for which 

 sodium vapour has refracfixe indices that are smaller than unity. 

 Such rays deviating towards the left (as shown in SJ^), are intercepted 

 by Q and consequently will be absent from the spectrum. 



Nos 6, 8 and 10 are reproductions of photographs taken under 

 these conditions. On the left are seen the smaller, on the right the 

 greater wave-lengths (in fact, in the whole series of photographs the 

 stronger Z)-line appears on the left side); so it is obvious that really 

 the waves lying on the red-facing side of the Z)-lines, i.e. those for 

 which the vapour has high refractive indices, are strengthened by 

 anomalous dispersion ; and that, on the other hand, the waves on 

 the violet side have been considerably ivealcened. 



Alternately with 6, 8 and JO the photographs 7, 9 and 11 were 

 taken. The position of the flame was now as indicated in Fig. 2, by 

 i. e. its axis had been shifted 3 mM. to the left, so that the central 

 beam had to traverse that part of the flame where the density of 

 the sodium vapour decreases from left to right. Here we notice that 

 the rays with low refractive indices deviate towards the right and 

 that a larger number of them reach the slit S^, e.g. S^K, whilsl the 

 rays with high refractive indices, such as S^G, are intercepted by Q. 



Nos 6 to 11 show the effect of a gradual increase in the density 

 of the sodium vapour. In No. 12 we again notice the sharply defined 

 sodium lines after the flame has been extinguished at the end of 

 the series of experiments ; they are somewhat stronger than those 

 at the beginning of the series, because much sodium vapour had 

 spread through the room during the operations. 



When carefully examining the original negatives it is possible in 

 most of them to distinguish the rather sharp central absorption lines 

 from the overlaying dispersion bands (especially in the photographs, 

 obtained when the position of the flame was symmetrical ; the repro- 

 ductions fail to bring out this peculiarity). Advantage has been taken 

 of this fact in so arranging the twelve photographs here reproduced, 

 that equal wave-lengths occupy corresponding places. Then it is seen 



