124 COSMOS. 



of the object-glass,) a mirror and diaphragms, whose rota- 

 tion is measured on a ring; telescopes with divided object- 

 glasses, on either half of which the stellar light is received 

 through a prism ; astrometers** in which a prism reflects the 



changement d'inclinaison d'un miroir entame sur la face in- 

 terieure. II blame d'ailleurs le principe de ma methode et le 

 regarde comme peu susceptible de perfectionnement, non seule- 

 ment a cause de la difference des angles entre 1'etoile vue 

 directement et celle qui est amenee par reflexion, mais surtout 

 parceque le resultat de la mesure d'intensite depend de la 

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

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

 limite inferieure de la portion non entamee du petit miroir." 

 " M. Arago, who possesses photometric data, differing entirely 

 from those hitherto published, 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 little susceptible 

 of correctness, not only on account of the difference of angles 

 between the star seen directly and by reflection ; but espe- 

 cially 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 observations when the pupil is not 

 exactly adjusted to the elevation of the lower limit of the un- 

 plated part of the small mirror." 



& Compare Steinheil, Elements der Helligkeits-Messungen am 

 Sternenhimmel, Munchen 1836, (Schum. Astr. Nachr. no. 609,) 

 and Sir J. Herschel, Results of Astronomical Observations made 

 during the years 1834-1838 at the Cape of Good Hope (Lond. 

 1847), pp. 353-357. Seidel attempted 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 

 legree of latitude in our northern latitudes. Assuming Vegt 

 tobe = l, he finds for Sirius 5-13; for Rigel, whose lustn 

 appears to be on the increase, 1'30 ; for Arcturus 0'84 ; for Ca- 

 pella 0-83 ; for Procyon 0'71 ; for Spica 0'49 ; for Atair 0'40 ; 

 for Aldebaran 0'36 ; for Deneb 0'35 ; for Regulus 0*34 ; for 

 Pollux 0-30 ; he does not give the intensity of the light of 

 Betelgeuze, on account of its being a variable star, as was parti- 

 cularly manifested between 1 836 and 1839. ( Outlines, p. 523.) 



