24 DR. VVOLLASTON'S METHOD OF COMPARING THE LIGHT 



Were the sun removed to such a distance, that the light which we received 

 from it were only a twenty thousand millionth part of its present light, that 

 distance would be equal to ^2 X 100,000 X its present distance, and it 

 would, if still situated in the ecliptic, have a parallax in longitude of nearly 

 3" ; but if placed at the same angular distance from the ecliptic, as Sirius, since 

 the parallax varies as the sine of a star's latitude, and the latitude of Sirius is 

 about 39°i, it would have a parallax in latitude of about 1"ts. 



Assuming the parallax of Sirius to be half a second, and consequently its di- 

 stance from the earth to be 525,481 times the distance of the sun from the 

 earth, Sirius, if placed at the sun's distance, would subtend 3.7 times the sun's 

 apparent diameter, and would afford us as much light as 13.8 suns. 



From similar experiments to those I made on Sirius, it appeared that the 

 light affoi-ded us by Lyra was about ig^opopoo*^ P^''*' ^^ t^^ sun's light, or about 

 ^th part of the light of Sirius. 



Without extending this method to a comparison of the stars with the sun, 

 we may confine it, if we think proper, to comparing the stars with one another, 

 so that, in fulfilment of the wishes of Mr. Michell*, " instead of distributing 

 them, as has hitherto been done, into a few ill-defined classes, they may be 

 ranked with precision, both according to their respective brightness, and the 

 exact degree of it." 



In concluding the paper which is now submitted to the Society, I request 

 them to direct their attention rather to the method than to the observations ; 

 for these have been much too few in number to enable me to state with any 

 degree of confidence, what proportion the light of the sun really does bear to 

 either of the stars compared with it. It was my intention, had my health per- 

 mitted it, to have proceeded with this inquiry, until by multiplied observations 

 I had ascertained how nearly the mean of one extensive series of comparisons 

 accorded with the mean of another series ; and how far, thei'efore, the method 

 itself was deserving of confidence. But since I have now no prospect of bring- 

 ing the subject to perfection, I submit the method itself to the consideration of 

 industrious observers, who will soon be able to judge of the expediency of con- 

 tinuing the inquiry. 



• Phil. Trans. 1767: p. 241. 



