492 M. A. J. Angstrom on a new Determination 



The difference of the wave-lengths corresponding to the two 

 D lines, as measured in the third and in the fourth spectrum, 

 amounts to 2*226, — that between the wave-lengths corresponding 

 to the two E lines being only 0*395, as measured in the third 

 spectrum. 



Fraunhofer has given two different series of values for the 

 wave-lengths of light. The first series was obtained by mea- 

 surements with wire gratings, and it is upon this that Cauchy 

 founded his calculations in the Memoire sur la Dispersion. 

 It contains the following numerical values (/3) : — 



T> r\ T\ "p p Q. TT 



2541, 2425, 2175, 1943, 1789, 1585, 1451. 



Comparing these values with the corresponding ones in the 

 foregoing Table, which I will call the series (a), the following 

 differences (u—/3) are obtained : — 



-1-3, +1-3, +3-6, +5-2, +8-3, + 7-4, +16-2. 



The differences increase, as will be seen, towards the violet 

 end of the spectrum, and are there very considerable. This 

 arises from the difficulty, when using gratings so coarse as those 

 employed by Fraunhofer, of accurately distinguishing the dark 

 lines at the violet end of the spectrum. 



The best of all the gratings employed by Fraunhofer is, with- 

 out doubt, that which he denoted as No. 4, and with which he 

 observed the line E even in the thirteenth spectrum. This grating 

 gives, in general, values which agree better with my own. For 

 the lines C, D, and E the agreement is nearly perfect. The 

 grating in question gave, in fact, the values 



t» p Ti t? -p" a tt 



2542, 2426, 2178, 1947, 1794, 1586, 1457. 



I conclude from this that the disagreement between the 

 series («) and (/3) must arise principally from errors of observa- 

 tion, which, with the wire gratings used by Fraunhofer, were 

 unavoidable. 



The other series of values of wave-lengths given by Fraun- 

 hofer is of a somewhat later date. It will be found in Gilbert's 

 Annalen der Physik, vol. lxxiv., as well as in Herschel's ' Optics/ 

 Schwerd's Beugungs-Erscheinungen, and other works. This series, 

 on account of its exactitude, appears to have been held by Fraun- 

 hofer in greater esteem than the older ones. 



It contains the following values (y) :— 



P T) T" T" C 1 TT 



2422, 2175, 1945, 1794, 1587, 1464; 

 and gives, when compared with the series (a), the differences 

 («-7): +4-3, +3-6, +3*2, +3*3, +5*4, +3-1. 



