PHOTOMETRIC METHODS. 93 



ing (iii front of the object-glass) a mirror and diaphragms, 

 whose rotation is measured on a ring ; telescopes with di- 

 vided object-glasses, on either half of which the stellar light 

 is received through a prism ; astrometers* in which a prism 

 reflects the image of the moon or of Jupiter, and concentrates 

 it through a lens at different distances into a star more or 

 less bright. Sir John Herschel, who has been more zealous- 

 ly engaged than any other astronomer of modern times in 

 making numerical determinations in both hemispheres of the 

 intensity of light, confesses that the practical application of 

 exact photometric methods must still be regarded as a " de- 

 above-mentioned mode of classification, be compared directly with 

 those which Sir John Herschel made public as early as 1838. (See my 

 Recneil d'Observ. Astr., vol. i., p. Ixxi., and Rclat. Hist, du Voyage aux 

 Regions Equin., t. i., p. 518 and 624; also Lettre de M. de Humboldt a 

 M. Schumacher en Fevr., 1839, in the Astr. Nachr., No. 374.) In this 

 letter I wrote as follows : " M. Arago, qui possede des moyens photo- 

 metriques entierement difierents de ceux qui ont ete publics jusqu'ici, 

 m'avait rassure sur la partie des erreurs qui pouvaient provenir du change- 

 ment d'inclinaison d'un miroir entame sur la face interieure. H blame 

 d'ailleurs le principe de ma methode et le regarde comme peu suscep- 

 tible de perfectionnement, non seulement a cause de la difference des 

 angles entre 1'etoile vue directement et celle qui est amenee par reflex- 

 ion, mais surtout parceque le resultat de la mesure d'intensite dfepend 

 de la partie de 1'ceil qui se trouve en face de 1'oculaire. II y a erreur 

 lorsque la pupille n'est pas tres exactement a la hauteur de la limite in- 

 ferieure de la portion non eutamee du petit miroir." " M. Arago, who 

 possesses photometric data differing entirely from those hitherto pub- 

 lished, had instructed me in reference to those errors which might arise 

 from a change of inclination of a mirror silvered on its inner surface. 

 He moreover blames the principle of my method, and regards it as lit- 

 tle susceptible of correctness, not only on account of the difference of 

 angles between the star seen directly and by reflection, but especially 

 because the result of the amount of intensity depends on the part of the 

 eye opposite to the ocular glass. There will be an error in the observ- 

 ations when the pupil is not exactly adjusted to the elevation of the 

 lower limit of the unplated part of the small mirror." 



* Compare Steinheil, Elemente der Helligkeils-Messungen am Sternen- 

 himmel Munchen, 1836 (Schum., Astr. Nachr., No. 609), and John Her- 

 schel, Results of Astronomical Observations made during the Years 1834 

 -1838 at the Cape of Good Hope (Lond., 1847), p. 353-357. Seidel at- 

 tempted in 1846 to determine by means of Steinheil's photometer the 

 quantities of light of several stars of the first magnitude, which attain 

 the requisite degree of latitude in our northern latitudes. Assuming 

 Vega to be =1, he finds for Sirius 5-13 ; for Rigel, whose luster appears 

 to be on the increase, 1-30; for Arcturus, 0-84; for Capella, 0-83; for 

 Procyon, 071; for Spica, 0-49; for Atair, 0-40; for Aldebaran, 0-36; 

 for Deneb, 0-35; for Regulns, 0-34; for Pollux, 0-30; he does not give 

 the intensity of the light of Betelgeux, on account of its being a varia- 

 ble star, as was particularly manifested between 1836 and 1839. (Ont 

 tiite*, p. 523 ) 



