penetrating into Space by Telescopes. 61 



be of the second magnitude, may be added to our experimental 

 knowledge; for, by this means, we are informed what we are 



to understand by the expressions — -r-, rr> „ = ?z * We 



J © iuriusj # launl 



cannot wonder at the immense difference between the bright- 

 ness of the. sun and that of Sirius ; since the two first expres- 

 sions, when properly resolved, give us a ratio of brightness of 

 more than 170 thousand millions to one; whereas the two 

 latter, as has been shewn, give only a ratio of four to one. 



What has been said will carry us, with very little addition, to 

 the end of our unassisted power of vision to penetrate into space. 

 We can have no other guide to lead us a third step than the 

 same beforementioned hypothesis ; in consequence of which, 

 however, it must be acknowledged to be sufficiently probable, 

 that the stars of the third magnitude may be placed about three 

 times as far from us as those of the first. It has been seen, by 

 my remarks on the comparative brightness of the stars, that I 

 place no reliance on the classification of them into magnitudes ;-f 

 but, in the present instance, where the question is not to ascer- 

 tain the precise brightness of any one star, it is quite sufficient 

 to know that the number of the stars of the first three different 

 magnitudes, or different brightnesses, answers, in a general way, 

 sufficiently well to a supposed equally distant arrangement of a 

 first, second, and third set of stars about the sun. Our third 

 step forwards into space, may therefore very properly be said to 

 fall on the pole-star, on y Cygni, e Bootis, and all those of the 

 same order. 



* The names of the objects Q, Sirius, (3 Tauri, are here used to express their dis- 

 tance from us. 



f Phi!. Trans, for the year 1796, page 168, 169. 



