niOTOMETRIC METHODS. 93 



ing (in 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 difierent 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-mentioncd mode of classification, be compared directly with 

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

 Recneil d'Observ. Ast):, vol. i., p. Ixxi., and Rclat. Hist, du Voyage avx 

 Regions Equln., t. i., j). 518 and 624; also Lettre de M. de Humboldt a 

 M. Schumacher en Few., 1839, in the Astr. Nackr., No. 374.) In tliis 

 letter I wrote as f<)llows : " M. Arago, qui possede des moyens plioto- 

 raetiiques entierement difFerents de ceux qui ont ete publics juiiqu'ici, 

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

 ment d^inclinaison d'\ui miroir entame sur la face interieure. II blame 

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

 tible de perfectionnenient. non seulement a cause de la difference des 

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

 ion, mais surtout parceque le resnltat de la mesure d'intensite depend 

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

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

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

 possesses photometric data differiiig 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 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 observ- 

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

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



* Compai'e Steinheil, Eleme7ite der HelHgkeUs-Messungen am. Sterncn- 

 himmel Munchen, 183G (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. 353-3.57. 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, whoso luster a})pears 

 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 Regubis, 0-34; for Pollux, 0-30; he does nt)t give 

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

 ble star, as wa** particularly manifested between 1836 and 1839. {Out 

 Unes, p. 523 ) 



