PHOTOMETRY. 127 



the full moon ; 73 whence it follows that the light transmitted 

 to us from the sun is to the light which we receive from a 

 Centauri as 22000 millions to 1 . It seems therefore very pro- 

 bable, when, in accordance with its parallax, we take into 

 account the distance of the star, that its (absolute) proper 

 luminosity exceeds that of our sun by 2 T 3 -Q times. Wollastoii 

 found the brightness of Sirius 20000 million times fainter 

 than that of the sun. From what we at present believe to be 

 the parallax of Sirius (O'"230) its actual (absolute) intensity of 

 light exceeds that of the sun 63 times. 74 Our sun there- 

 fore belongs, in reference to the intensity of its process of 

 light, to the fainter fixed stars. Sir John Herschel esti- 



73 Wollaston in the Philos. Transact, for 1829, p. 27. 

 Herschel' s Outlines, p. 553. Wollaston's comparison of the 

 light of the sun with that of the moon was made in 1799, and 

 was based on observations of the shadows thrown by lighted 

 wax tapers, while in the experiments made on Sirius in 1826 

 and 1827, images reflected from thermometer bulbs were em- 

 ployed. The earlier data of the intensity of the sun's light, 

 compared with that of the moon, differ widely from the results 

 here given. They were deduced by Michelo and Euler, from 

 theoretical grounds at 450000 and 374000, and by Bouguer 

 from measurements of the shadows of the light of wax tapers, 

 at only 300000. Lambert assumes Venus, in her greatest inten- 

 sity of light, to be 3000 times fainter than the full moon. Ac- 

 cording to Steinheil, the sun must be 3286500 times further 

 removed from the earth than it is, in order to appear, like Arc- 

 turus, to the inhabitants of our planet (Struve, Stellarum Com- 

 positarum Mensurce Micrometricce, p. clxiii.) ; and according to 

 Sir John Herschel the light of Arcturus exhibits only half the 

 intensity of Canopus ; Herschel, Observ. at the Cape, p. 34. 

 All these conditions of intensity, more especially the impor- 

 tant comparison of the brightness of the sun, the full moon, and 

 of the ash-coloured light of our satellite which varies so greatly 

 according to the different positions of the earth considered as 

 a reflecting body, deserve further and serious investigation. 



74 Outl. ofAstr., p. 553 ; Astr. Observ. at the Cape, p. 363. 



