( 355 ) 



to see light emitted by the vapour, in which light different wave- 

 lengths occur, all greater than the exact wave-length of the serial 

 lines. The observed displaced lines of the second secondary series 

 are consequently comparableto appai-cnt emission lines of the spectrum 

 d of our plate I. 



In this explanation things have been represented as if the light of 

 these serial lines had to be exclusively attributed to anomalous dis- 

 persion. Probably however in the majority of cases emission proper 

 will indeed perceptibly contribute to the formation of the line; the 

 sharp edge must then appear in the exact place belonging to the 

 particular wave-length. 



How can we now explain that lines of other series are diffuse at 

 the opposite side? Also this may be explained as the result of ano- 

 malous dispersion if we assume that of the emission centres of these 

 other series the density increases when we move away from the 

 positi\e carbon point. In this case namely the rays originating in 

 the crater, which are concave towards the carbon point and conse- 

 quently seem to come from the arc, possess shorter wave-lengths 

 than the serial lines, i. e. the serial lines appear widened towards 

 the violet. This supposition is not unlikely. For the positive and 

 negative atomic ions which according to Stark's theory are formed 

 in the arc by the collision of negative electronic ions, move in opposite 

 directions under the influence of the electric field; hence the density 

 gradients will have opposite signs for the two kinds. Series whose 

 lines are diffuse towards the red and series whose lines flow out 

 towards the violet would, according to this conception, belong to 

 atomic ions of opposite signs — a conclusion which at all events 

 deser\es nearer investigation. 



The examples given may suffice to show that it is necessary syste- 

 matically to investigate to what extent the already known spectral 

 phenomena may be the result of anomalous dispersion. A number 

 of cases in which the uutil now neglected principle of ray-curving 

 has undoubtedly been at the root of the matter are found in Kayser's 

 handbook II, p. 292—298, 304, 306, 348-351, 359—361, 366. 



Dispersion hands in the spectra of celestial bodies. 



Since almost any peculiarity in the appearance of spectral lines 

 may be explained by anomalous dispersion if only we are at liberty 

 to assume the required density distributions, we must ask when 

 applying this principle to astrophysical phenomena : can the values 

 of the density gradient for the different absorbing gases in celestial 



