•penetrating into Space by Telescopes. 57 



such as I have made, are raised, where the word brightness is 

 used as absolute, when we should have kept it to the only meaning 

 it can bear in the mathematicians' theorem. 



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 farther, 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 a hundred times less 

 than on the earth ; and that consequently, though it may there 

 be intrinsically as bright, it must here be absolutely* an hundred 

 times brighter. 



The next objection I have to consider, relates to the fixed 

 stars. What has been shewn in the preceding paragraph, with 

 regard to the sun, is so intirely 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 illu- 

 mination 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 



* See the definition of absolute brightness, page 52. 

 MDCCC I 



