512 



REVERSION OF BRIGHT LINES. 



the eye is properly adjusted with reference to its distance from the 

 slit. For this adjustment place a needle horizontally across the slit 

 and fix it with a bit of wax. Look at the spectrum of the paraffin 

 lamp ; it will be divided through its whole length into two portions 

 by a black horizontal line ; when the eye is so placed that this hori- 

 zontal line is seen quite distinctly, it is also in the right position for 

 seeing clearly the vertical sodium line. 



This mode of reversing the sodium line is not the 

 same as that previously described for the reversion of 

 the solar or lime light spectrum. When the spirit 

 flame is placed between the paraffin flame and the slit, 

 instead of between the prism and the eye, the sodium 

 line is not dark ; it is indeed much brighter than the 

 spectrum of the paraffin flame which is seen at the 

 same time. The reason of this is that the quantity of 

 yellow light emitted by the paraffin lamp which is 

 absorbed in its passage through the spirit flame is much 

 less than the quantity of yellow light which the spirit 

 flame emits itself. On the other hand, when the flame 

 is between the prism and the eye, it does not produce a 

 yellow line, but illuminates equally every portion of 

 the spectrum behind it, so that the part which cor- 

 responds to the sodium line receives the same light 

 as every other part of <the spectrum; hence, if the 

 sodium vapour were equally transparent for every part 

 of the spectrum of the paraffin flame, no change what- 

 ever would be observed in any part of the spectrum. 

 But what actually takes place is that the sodium vapour 

 in the spirit flame partly stops the light emitted by a 

 particular yellow part of that spectrum ; there appears, 

 therefore, in that place a line which is the darker the 

 more the sodium vapour absorbs of the light given out 

 by the corresponding part of the original spectrum. 



