rilOTOMETRIC METHODS. 1)3 



ing (in front of the object-glass) a mirror and diapliragms, 

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

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

 is received throngh 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-menlioned mode of classification, be compared directly with 

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

 Recueil cC Observ. Astr., vol. i., p. Ixxi., and Rclat. Hist, cht Voyage anx 

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

 M. Schumacher en Fcvr., 1839, in the Astr. Kachr., No. 374.) In this 

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

 metriques entierement diflfereuts de ceux qui ont ete publies jusqu'ici, 

 m'avait rassure siirla partie des erreurs qui pouvaient pi'ovenir du change- 

 menfd'iuclinaison d'un miroir entame sur la face interieure. II blame 

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

 tible de perfectiomiement, iion seulement a cause de la difference des 

 angles eutre I'etoile vue directement et celle qui est amenee par reflex- 

 ion, mais surtout parceque le resultat de la niesure d'iutensite depend 

 de la partie de I'ceil qui se trouve en face de I'oculaire. II y a errenr 

 lorsque la pupille n'est pas tres exactement k la hautem* de la limite in- 

 ferieure de la portion uou entamee du petit miroir." " M. Arago, who 

 possesses photometric data differing entirely from those hitherto pub 

 lished, had instmcted me in x'eference 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 

 Bngles between the star seen dii-ectly and by reflection, but especially 

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

 eye opposite to tlie ocular glass. There will be an error in the obsen'- 

 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 HeUigkeits-Messnngen am S/cr nen- 

 jimviel Mimchen, 1836 (Sebum., Astr. Nachr., No. 609), and John Her- 

 eche], Results of Astronomical Observations made during the Ycarx 1834 

 -1838 at the Cape of Good Hope (Lend., 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 Sinus 5-13 ; for Rigel, whose luster appears 

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

 Procyon, 0-71; for Spica, 0-49; for Atair, 0-40; for Aldebaran, 0-36; 

 for Deneb, 0-35; for Regains, 0-34; for Pollux, 0-30; he does not giva 

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

 ble star, as was particularly manifested between 1836 and 1S39. {Out 

 iines, p. 5J3 ) 



