216 



Professor Dewar 



[June 65 



however, only well defined in crucibles having a height of at least 

 3 inches above the arc. 



On putting a fragment of metallic gallium into a crucible, the less 

 refrangible line, wave-length 4,170, came out bright, and soon a dark 

 line appeared in the middle of it. The other line, wave-length 4,031, 

 showed the same effect, but less strongly. 



Reviewing the series of reversals which have been observed, in 

 many cases the least refrangible of binary groups is the most easily 

 reversed, as has been previously remarked by Cornu. 



Making a general summation of the results respecting the alkaline 

 earth metals, potassium and sodium, having regard only to the 

 most characteristic rays, which for barium may be taken as 21, for 

 strontium 34, for calcium 37, for potassium 31, and for sodium 12, 

 the reversals number respectively 6, 10, 11, 13, and 4. That is in 

 the case of the alkaline earth metals about one-third, and these 

 chiefly in the more refrangible third of the visible spectrum, the 

 characteristic rays remaining unreversed in the more refrangible 

 part of the spectrum being respectively 2, 5, and 4. 



The curious behaviour of the lines of different spectra with regard 

 to reversal induced a comparison with the bright lines of the chromo- 

 sphere of the sun, as observed by Young. It is well known that some 

 of the principal lines of the metals giving comparatively simple 

 spectra, such as lithium, aluminium, strontium, and potassium, are not 

 represented amongst the dark lines of Fraunhofer, while other lines 

 of those metals are seen : and an examination of the bright chromo- 

 spheric lines shows that special rays highly characteristic of bodies 

 which appear from other rays to be present in the chromosphere are 

 absent, or are less frequent in their occurrence than others. 



In the following table the relation between the observations on 

 reversals and Young's on the chromospheric lines is shown. 



