BIBLIOGRAPHY OF IIEKSCIIEL's WRITINGS. 565 



Herschel W.: Byxopsis of tiik Wiutings of — CVmlinned. 



A. D. Vol. P. 



1800 90 55 Objection to tLc foregoing. I. being proved that an object is equally 

 bright at all distances, it may be urged that in a telescope the dif- 

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

 brightness, and that we must consequently bo able to see stars 

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

 5G The origin of such objectious 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 ojjticians with regard to what I call intrinsic 



briglituess, will not oppose what I affirm of absolute brightness. 



57 Though the sun, to an observer on Saturyi, must be as bright as it is 



here ou 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 eartli receives when he sees it. 

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

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

 omer, but it should be remembered that the sun ou Saturii appears 

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

 quently, though it may there be intrinsically as l)right, it must here 

 be absolutely an hundred times brighter. 



57 This reasoning is entirely applicable to the stars; and the light we 



aH 

 can receive from the stars is truly expressed by -p^- 



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- 

 in"-, 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 Georgian Planet, it has never 

 been allowed us to perceive. 



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



bly more extended, but less accurately to be ascertained. 



59 The'o-eneral supposition is admitted that stars, at least those which 

 seem to be promiscuously scattered, are probably one with another 

 of a certain magnitude. 



59 [Foot-note.] The PMl Trans, for the year 1796, page 166, 10/, 168 is 

 referred to for support of this assumption. 

 The difference in briglituess between Sirius, Arcturus, a Cygni, and 

 /3 Tanri, does not seem to alter the dimensions of the iris; a, there- 

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

 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 he 

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

 Our third step forward into space may therefore very properly he 

 said to fall on the pole star, on r Cygni, e Bootis, and all those of 



the same order. ^^a^t ia 



The difterence between these and the stars of the rreceding 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 



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