the Waves of certain bright Lines in the Spectrum. 261 

 For the blue strontium line Sr 8 I obtained 



SrSl x= 5 54, and hence X=0-0002056 

 Sr32 y=li 49, „ X=Q-Q002048 

 The mean therefore is X= 0*0002052 



or \=0-0004631 millim. 



The setting the instrument on the blue strontium line involved 

 some difficulty, owing to its little intensity. 



The attempt to determine in the same manner the length of 

 wave of the two potassium lines was not successful, because they 

 were too weak even to be seen at all. 



To this communication I subjoin the remark that the bright 

 spectrum-lines of certain coloured flames appear very appropriate 

 for determining the index of refraction of various substances for 

 the differently coloured rays. Hitherto, as we know, Fraunho- 

 fer's lines A, B, C, D, E, F, G, and H have been employed for 

 this purpose. 



The observation of the bright spectral lines of coloured flames 

 is at all events easier of application, and may be had recourse to 

 at any time, whereas the employment of Fraunhofer's lines is 

 dependent on the weather, and requires a suitable locality. As 

 long ago as 1859, Pliicker suggested for the determination of 

 the index of refraction the employment of the three bright lines 

 obtained by the passage of the spark from a Ruhmkorff's coil 

 through rarefied hydrogen, instead of Fraunhofer's lines. And 

 Landolt has already carried this into effect with respect to a series 

 of different fluids (PoggendorfFs Annalen, vol. cxvii.). But it is 

 even yet simpler and less troublesome to make use of a Bunsen's 

 burner than to put a RuhmkorfPs coil in action. The lines best 

 suited for the end in view would probably be 



The red potassium line . . Ka «. 



The red lithium line . . . Li «. 



The yellow sodium line . . Na a. 



The blue strontium line . . Sr 8. 



The violet potassium line. « Ka/3. 



To which might probably be added the green thallium line when 

 this metal shall have become somewhat more accessible. All 

 these lines, owing to their great distinctness, are very suitable 

 for accurate determinations. 

 Freiberg, March 1863. 



I am happy to have it in my power to annex already to the 

 above notices the length of wave of the green thallium line. 

 Professor von Babo, during the course of the last vacation, 



