BIBLIOGRAPHY OF ITEKSCIIEL's WRITINGS. 565 



Herschel, W.: Syxopsis of the Wiutings of— Coutiiiucd. 



A. D. Vol. p. 



1800 90 55 Olijpfition to the foregoing. II ln'ing proved that an ol)ject is equally 

 bright at all distances, it may be nrgcni tiiat in a telescope the dif- 

 ferent distance of stars can be of no account witli regard to their 

 brightness, and that we must eonse<inently bo able to see stars 

 which are many thousands of times farther than Sirius from us. 

 56 The origin of such objections is in want of distinction in the two sorts 

 of brightness, which have been discriminated by intrinsic and abso- 

 lute brightness. 



56 The demonstrations of opticians with regard to what 1 call intrinsic 



brightness, will not oppose what I affirm o{ absolute brightness. 



57 Though the sun, to an observer on Saturn, must bo as bright as it is 



here on the earth, it 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 lie sees it. 

 57 The picture of the sun on the retina of the Saturnian observer is as 

 intensely illuminated as that on the retina of the terrestrial astron- 

 omer, l)nfc it should be remembered that the sun on Saturii appears 

 to be a hundred times less than on the earth; and that conse- 

 quently, though it may there be intrinsically as bright, it must here 

 be absolutely an hviudred times brighter. 



57 This reasoning is imtirely applicaltle to the stars; and the light we 



can receive from the stars is truly exjjressed by v^j- 



58 Hence I am authorized to conclude 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 speak- 

 ing, very soon invisible with our best instruments. 



58 With respect to the naked eye,. the power of penetrating into space 



is limited. Among reflecting luminous objects our penetrating 

 powers are sufficiently ascertained. An object seen by reflected 

 light at a greater distance than the Gcoryian Planet, it has never 

 been allowed us to i:)erceive. 



59 The range of natural vision with self-luminous objects is incompara- 



bly more extended, but less accurately to be ascertained. 

 59 The general supposition is admitted that stars, at least those which 

 seem to be promiscuously scattered, are prol)ably one with another 

 of a certain magnitude. 



59 [Foot-note.] The Phil. Trans, for the year 1796, page 166, 167, 168 is 



referred to for support of this assumption. 



60 The difference in brigiitness between Sirius, Arcturus, a Cygni, and 



/i Tauri, does not seem to alter the dimensions of the iris ; a, there- 

 fore, becomes a given quantity and may be left out. 



61 Admitting that the latter of these stars are probably at double the 



distance of the former, we can have no other guide to lead us a 

 third step than the before-mentioned hypothesis ; in consequence of 

 which it is probable that stars of the third magnitude may be 

 placed about three times as far from us as those of the iirst. 



61 Our third step forward into space may therefore very properly be 



said to fall on the polo star, on ;' Cygni, £ Boutis, and all those of 

 the same order. 



62 The difference between these and the stars of the preceding order is 



much less striking than that between the stars of the first and sec- 

 ond magnitudes. So the calculated ratio of the brightness of a star 



