"Dr 0\hevs on the Transparency of Space. 147 



Log 799 = 2.9025467793 

 Log 800 = 2.9030899870 



a = 0.0005432077 

 Therefore log a = 6. 7349604 — 10. 



It is easy, again, to calculate the diminution of brightness of 

 stars for more considerable distances. 



Let us now suppose the lustre A of a star, such as our sun, 

 but placed at the distance of Sirius, which we took a little ago 

 for unity, itself equal to 1 ; the lustre of this star will be, 



jQ at a distance equal to 84.23 times that of Sirius. 

 A 178.40 



/s 285.16 



/o 408.41 



A 554.13 



We see, therefore, that, at the extreme distances at which our 

 armed eye can still distinguish isolated stars, the lustre is di- 

 minished by one-half. The absolute brightness of stars may 

 establish between them differences equally remarkable and still 

 greater. 



The lustre must not be confounded with the intensity of the 

 %ht. 



This intensity is the lustre multiplied by the apparent mag- 

 nitude : it is directly proportional to the lustre, and, inversely, 

 the square of the distance. Thus, a star 554 times more distant 

 from us than Sirius, has still the half of the lustre, but only 

 ^^i^_th of the luminous intensity of that star. 



The lustre diminishes considerably at greater distances. At 

 a distance equal to 1842.9 times that of Sirius, it is only j^^th 

 of the lustre of that star; at the distance of 3681.8 it is not 

 more than yj^; and at that of 5522.7, it is xo\o> ^^^ ^ i^ 

 proportion. 



At what distance would the light of a fixed star still have the 

 lustre of the full moon, supposing this lustre to be ^oo^ous? ®^ 

 that of the sun ? As we have, then *, 



• Here — zz ^ is substituted in the equation, log — = — a ^, or 



log X - log(log^)~. log a. 



K 2 



