246 Mr. Louis Bell on the Absolute 



definite standards of reference, and because the apparatus was 

 as yet very defective. Fraunhofer's research, " Bestimmung 

 des Brecbungs- und Farban-zerstr 'unngs-Vermogens ver- 

 schiedener Grlasarten," was presented to the Munich Academy 

 of Sciences in 1814, and was published in the fifth volume 

 of the Denkschriften. It then became possible to study in 

 detail the properties of rays of definite position, and the 

 work was taken up almost immediately. Almost the first 

 step was to determine the wave-lengths of prominent points 

 in the solar spectrum ; and, as is well known, Fraunhofer 

 himself took it, determining the wave-lengths corresponding 

 to his lines B, C, D, E, F, Gr, H. As there seems to have 

 been (noticeably in Verdet's papers) some confusion concern- 

 ing his papers on this subject, it may be well here to clear 

 the matter up. 



Fraunhofer's first paper dealing with the subject was pre- 

 sented to the Munich Academy in 1821. It is entitled, 

 "Neue Modification des Lichtes durch gegenseitige Einwir- 

 kung und Beugung der Strahlen, und Gresetze derselben," 

 and was printed in the eighth volume of the Denkschriften. 

 It is of considerable length, and deals with various diffraction 

 phenomena, but its chief interest lies in the wave-length 

 measurements made with wire gratings. The experiments 

 made with ten of these are given in detail, and are remarkably 

 careful and consistent. The gratings were quite various, the 

 wires being from 0'04 to Ob' millim. in thickness, and the 

 grating-space, as ordinarily measured, from O0528 to 0*6866 

 millim. From these proportions it is evident enough that 

 the spectra must have been imperfect ; but, in spite of this, 

 Fraunhofer obtained results which agreed remarkably well 

 with each other. The wave-lengths of D, as obtained from 

 the above-mentioned ten gratings, were as follows, reduced to 

 millimetres : — 



millira. millim. 



(1) 0-0005801 



(2) 0-0005894 



(3) 0-0005891 



(4) 0-0005897 



(5) 0-0005885 



(6) 00005888 



(7) 0-0005885 



(8) 0-0005885 



(9) 0-0005882 

 (10) 0-0005882 



The mean value adopted was 0-0005888 millim., which, 

 considering the gratings, and the fart that most of the angles 

 of deviation were less than 1°, is certainly remarkably accu- 

 rate. It should be noted, too, that the liner gratings (1) to 

 (4) gave even better results. 



A brief discussion of this paper appeared in the seventy- 



