of the Lengths of Waves of Light. 495 



tends to increase the probability of the existence of an error in 

 the value of e corresponding to the finer grating. 



The values of the wave-lengths obtained by means of Nobert's 

 grating, therefore, appear to me to merit a greater confidence 

 than that which Fraunhofer's can justly claim. 



II. 



As already stated at the commencement of this paper, I have 

 not limited my measurements to the principal lines of Fraun- 

 hofer. I have measured, with the circle, the angle © for all the 

 stronger lines at a distance from each other of from 10' to 20', 

 and determined with the eyepiece-micrometer the positions of 

 the remaining intermediate lines. The measurements, moreover, 

 were repeated in the second, third, and fourth spectra, in order 

 to verify their exactitude. 



The following Table contains some of these results, those wave- 

 lengths alone being given which correspond to the strongest and 

 most prominent lines of the solar spectrum. Most of these 

 lines belong to iron or to lime, and have consequently a double 

 interest, since they present themselves also in the gas-spectra of 

 these substances. In order to give the reader a visible image of 

 the position and breadth of these lines in the solar spectrum, 

 I have added a figure (Plate III. fig. 1), which correctly shows 

 their respective positions as presented by a prism of sulphide of 

 carbon having an angle of 60°. An arc of 2' corresponds in the 

 figure to a length of one millimetre. 



Table II. — Wave-lengths, in hundred millionths ( = i^) of a 

 Paris inch. 



Line. 



Wave- 

 length. 



Spectra in which cor- 

 responding lines are 

 observed. 



Remarks. 



{ 



2812 



25397 



2426-29 



2312-2 



2287-3 



2279-6 



2276-8 



2269-4 



22677 



22621 



2255-1 



2179-70 



2177-48 



2076-1 



2071-3 



2069-7 



2068-3 



2065-4 



Earth's atmosphere ... 



Iron and calcium 



Iron. 

 Iron 



Strong line. 



Group of strong lines. 



Two groups of lines. 



