'PHOTOMETRY. 95 



Sir John Herschel has endeavored to determine the rela- 

 tion between the intensity of solar light and that of a star of 

 the first magnitude by a photometric comparison of the moon 

 with the double star a Centauri of the southern hemisphere, 

 which is the third in brightness of all the stars. He thus 

 fulfilled (as had been already done by Wollaston) a wish ex- 

 pressed by John Michell* as early as 1767. Sir John Her- 

 schel found from the mean of eleven measurements conduct- 

 ed with a prismatic apparatus, that the full moon was 27,408 

 times brighter than a Centauri. According to Wollaston, the 

 light of the sun is 80 1 ,072 times brighter than the full moon ;f 

 whence it follows that the light transmitted to us from the 

 sun is to the light which we receive from a Centauri as 

 22,000 millions to 1. It seems, therefore, very probable, 

 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 times. Wollaston found the 

 brightness of Sirius 20,000 million times fainter than that of 

 the sun. From what we at present believe to be the paral- 

 lax of Sirius (0"230), its actual (absolute) intensity of light 

 exceeds that of the sun 63 times.J Our sun therefore be- 

 longs, in reference to the intensity of its process of light, to 

 the fainter fixed stars. Sir John Herschel estimates the in- 

 tensity of the light of Sirius to be equal to the light of nearly 



* Philos. Transact., vol. lvii., for the year 1767, p. 234. 



t 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 employed. 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 theoret- 

 ical grounds, at 450,000 and 374,000, and by Bouguer, from measure- 

 ments of the shadows of the light of wax tapers, at only 300,000. Lam- 

 bert assumes Venus, in her greatest intensity of light, to be 3000 times 

 fainter than the full moon. According to Steinheil, the sun must be 

 3,286,500 times further removed from the earth than it is, in order to 

 ippear like Arcturus to the inhabitants of our planet (Struve, Stellarum 

 Compositarvm Mensurce Micromctricm, p. clxiii.); and, according to 

 Sir John Herschel, the light of Arcturus exhibits only half the intensity 

 of ZJanopus. Herschel, Observ. at. the Cape, p. 34. All these conditions 

 of intensity, more especially the important comparison of the bright- 

 ness of the sun, the full moon, and of the ash-colored light of our satel- 

 lite, which varies so greatly according to the different positions of the 

 earth considered as a reflecting body, deserve further and serious in- 

 vestigation. 



X Outl. of AsVr., p. 553 ; Astr. Observ. at the Cape, p. 363. 



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