32 THE CHEMISTRY OF THE SUN. [CHAP. 



of the slit, in different parts of the spectrum. We do not get 

 the complete spectrum represented by 



Wll FE) (rb W /pj\ no) 

 LI LM5 vl^i Li '^J LTA 



but, in the case of sodium, only 



Y 



lithium, only 



Fo) 



Lr\l 



with a substance which gives us a complicated spectrum we 



see lines, as under, here and there ; the light in each of these 



cases assuming in the telescope the form of the slit, and occupying 



different positions along the spectrum according to the light 



FIG. 15. The spsctrum of a complicated light-source as seen with a circular and 



a line slit. 



source. Here then we are in possession of the bright line 

 phenomenon first so called by Fraunhofer. 



Now why do we see lines ? For this reason 

 The light passing through the lenses forms an image of the slit 

 because in this case the prism deviates the light without dispersing 

 it, and we should see an image of the slit if we used the collimator 

 and telescope alone without the prism. 



Now apart from the difficulty of constructing fanciful forms 

 of slits there is one very powerful reason that leads to the pre- 

 ference of the line above all other forms. So long as we are using 

 a high dispersion and dealing only with bodies that give out 

 light of very few coloufs, it will not make very much difference 



