L. Bell — Absolute Wave-length of Light. 271 



1864 Ditscheiner. Ber. d. Wien. Acad , 1, Heft 2, 296. "Bestimmung der Wel- 

 le n lange der Fraunhofer'schen Linien des Sonnenspec- 

 trurjQS." 



1864 Angstrom. Pogg. Ann., cxxiii, 489. (Efvers. af Forhandl. (1863) 41. 

 "Neue Bestimmung der Lange der Lichtwellen nebst eine 

 Methods auf Optischen Wege diefortschreitende Bewegung 

 des Sonnensyslems zu bestimmen." 



1866 Ditscheiner. Ber. d. Wien. Acad., Hi, Heft 2, 289. " Eine absolute Bestim- 

 mung der Wellenlange der Fraunhofer'schen D Linien." 



1866 Maseart. Ann. de l'Ecole normale, iv, 7. : 'B,echerches sur la determi- 



nation des longueurs d'onde." 



1868 Maseart. Ann. de Chim. e. d. Phys., IT, xhi, 186. "Note sur differ- 



ents travaux relatifs anx longueurs d'onde." 



1868 van der WilligeD. Arch, du Musee Teyler, i, 1, 57, 280. " Memoire sur 

 la Determination des longueurs d'onde du Spectre Solaire." 



1868 Augstrom. TJpsala, 1868. " Recherches sur le Spectre Solaire." 



1871 Ditscheiner. Ber. d. Wien. Acad., lxiii, heft 2, 265. " Zur Bestimmung 

 der Wellenlange der Fraunhofer'schen Linien." 



1879 Peirce. Am. Jour. Sci., Ill, xviii, 51. '"Note on the Progress of Ex- 



periments for comparing a Wave length with a Metre." 



1884 Thalen. TJpsala, 1885. "Sur le Spectre du Fer obtenu a l'aide de 



1'arc electrique," p. 18. 



1886 Miiller and Kempf. Publicationen des Astrophysikalischen Observatoriums 

 zu Potsdam, v. '•Bestimmung der Wellenlange von 300 

 Linien im Sonne u spectrum." 



1886 de Lepinay. Jour, de Ph., II. v, 411. ■' Determination de la valeur absolue 



de la longueur d'onde de la raie D 2 ." 



1887 Bell. Am. Jour. Sci, III, xxxiii, 167. "On the absolute Wave 



length of Light. " 

 1887 Kurlbaum. Berlin, 1887. "Bestimmung der Wellenlange einiger Fraun- 

 hofer'scheu Linien." 



In general the determination of absolute wave-length involves 

 two quite distinct problems— first the precise determination of 

 some quantity which is an exact function of the wave-length 

 and some other linear dimension ; and second, the reduction 

 of this dimension to terms of some recognized standard of length. 

 The first process can be made to give relative wave-lengths with 

 a very high degree of accuracy, and is, in nearly every case, 

 more exact than the second, which constitutes the main diffi- 

 culty of the investigation. It is because the diffraction grating 

 lends itself readily to linear measurement, that its use is prefer- 

 able to the other interference methods which involve, usually, 

 the exact determination of a single very small linear quantity. 

 The ingenious attempt of M. de Lepinay* to avoid this difficulty 

 is interesting theoretically but practically it involves a quantity 

 even more uncertain than the average standard of length — the 

 relation between the kilogram and the meter — to say nothing 

 of the experimental difficulties of the method. The angular 

 measurements of nearly all the later investigators have been 

 quite good enough to furnish very exact values of wave-length, 

 but in every case it has been the measurement of the grating 

 space that has produced the manifold errors and discrepancies 

 * Journ. Phys.. II, v, 411. 



