▼OL. XC.J PHILOSOPHICAL TRANSACTIONS. 585 



The first objection I have mentioned is, that the sun, to an observer on Saturn, 

 must be as bright as it is here on earth. Now by this cannot be meant, that an 

 inhabitant standing on the planet Saturn, and looking at the sun, should absolutely 

 receive as much light from it as one on earth receives when he sees it; for this 

 would be contrary to the well-known decrease of light at various distances. The 

 objection therefore can only go to assert, that the picture of the sun, on the retina 

 of the Saturnian observer, is as intensely illuminated as that on the retina of the 

 terrestrial astronomer. To this I perfectly agree. But let those who would go far- 

 ther, and say that therefore the sun is absolutely as bright to the one as to the 

 other, remember that the sun on Saturn appears to be 100 times less than on the 

 earth; and that consequently, though it may there be intrinsically as bright, it 

 must here be absolutely 100 times brighter. 



The next objection relates to the fixed stars. What has been shown in the pre- 

 ceding paragraph, with regard to the sun, is so entirely applicable to the stars, that 

 it will be very easy to place this point also in its proper light. As I have assented 

 to the demonstration of opticians with regard to the brightness of the sun, when 

 seen at the distance of Saturn, provided the meaning of this word be kept to the 

 intrinsic illumination of the picture on the retina of an observer, I can have no 

 hesitation to allow that the same will hold good with a star placed at any assignable 

 distance. But I must repeat, that the light we can receive from stars is truly ex- 

 pressed by —^ and that therefore their absolute brightness must vary in the in- 

 verse 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 7 or 8 times farther from us than Sirius; and that they 

 become, comparatively speaking, very soon invisible with our best instruments. It 

 will be shown hereafter, that the visibility of stars depends on the penetrating power 

 of telescopes, which I must repeat falls indeed very short of showing 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 infinitely 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 confined 

 by nature, but is also 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 see borrowed illumination to the amazing 

 distance of more than 18 hundred millions of miles; especially when that light, in 

 vol. xviii. 4 F 



