58 Dr. Herschel on the Power of 



t, t 



the light we can receive from stars is truly expressed by ; 



and that therefore their absolute brightness must vary in the 

 inverse ratio of the squares of their distances. Hence I am 

 authorised to conclude, and observation abundantly confirms it, 

 that stars cannot be seen by the naked eye, when they are more 

 than seven or eight times farther from us than Sirius ; and that 

 they become, comparatively speaking, very soon invisible with 

 our best instruments. It will be shewn hereafter, that the visi- 

 bility of stars depends on the penetrating power of telescopes, 

 which, I must repeat, falls indeed very short of shewing stars 

 that are many thousands of times farther from us than Sirius ; 

 much less can we ever hope to see stars that are all but infi- 

 nitely distant. 



If now it be admitted that the expressions we have laid down 

 are such as agree with well known facts, we may proceed to 

 vision at a distance ; and first with respect to the naked eye. 



Here the power of penetrating into space, is not only con- 

 fined by nature, but is moreover occasionally limited by the 

 failure in brightness of luminous objects. Let us see whether 

 astronomical observations, assisted by mathematical reasoning, 

 can give us some idea of the general extent of natural vision. 

 Among the reflecting luminous objects, our penetrating powers 

 are sufficiently ascertained. From the moon we may step to 

 Venus, to Mercury, to Mars, to Jupiter, to Saturn, and last of 

 all to the Georgian planet. An object seen by reflected light at 

 a greater distance than this, it has never been allowed us to 

 perceive ; and it is indeed much to be admired, that we should 



