500 Ptuetraiion into Space hy Telejcoptj, 



at the dlflance of Saturn, or dill farther from us, mud be as bright as it is in its prefent 

 fituation. Nay, it may be urged, that in a telefcope, the different diftance of ftars can be 

 of no account with regard to their brightnefs, and that we mull confequently be able to 

 fee ftars which are many thoufands of times farther than Sirius from us ; in fliort, that a 

 ftar muft be infiaitely diftant not to be feen any longer. 



Now, objeclions fuch as thcfe, which feem to be the immediate confequence oT what has 

 been demonftrated by mathematicians, and which yet apparently contradicSl: what I affert in 

 this paper, deferve to be thoroughly anfwered. 



It may be remembered, that I have diftlnguifhed brightnefs into three different forts *. 

 Two of thefe, which have been difcriminated by intrinfic and abfoltite brightnefs, arc, in 

 common language, left without diftindlion. In order to (liew that they are fo, I might 

 bring a variety of examples from common converfation ; but, taking this for granted, it 

 may be fliewn that all the objections I have brought againft my theory have their founda- 

 tion in this ambiguity. 



The demonftrations of opticians, with regard to what I call intrinfic brightnefs, will not 

 oppofe what I affirm of abfolute brightnefs •, and I fhall have nothing farther to do than to 

 fnew that what mathematicians have faid, muft be underftood to refer entirely to the in- 

 trinfic brightnefs, or illumination of the pi£lure of objefts on the retina of the eye 5 from 

 which it will clearly follow, that their do£lrine and mine are perfectly reconcilable; and 

 that they can be at variance only when the ambiguity of the word brightnefs is overlooked, 

 and objections, fuch as I have made, are raifed, where the word brightnefs is ufed as abfo- 

 lute, when we fhould have kept it to the only meaning it can bear in the mathematicians' 

 theorem. 



The firft obje£lion I have mentioned is, that the fun, to an obferveron Saturn, muft be 

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

 on the planet Saturn, and looking at the fun, fliould abfolutely receive as much light from 

 it as one on earth receives when he fees it j for this would be contrary to the well known 

 decreafe of light at various diftances. The objeftion, therefore, can only go to affert, that 

 the pifture of the fun, on the retina of the Saturnian obferver, is as intenfely illuminated as 

 that on the retina of the tcrreftrial aftronomer. To this I perfecSlly agree. But let thofe 

 who would go farther, and fay that therefore the fun is ahfolutcly as bright to the one as to 

 the other, remember that the fun on Saturn appears to be a hundred times lefs than on the 

 earth ; and that confequently, though it may there be intrinfically as bright, it muft here be 

 abfolutely f an hundred times brighter. 



The next objection I have to confider, relates to the fixed ftars. What has been fliewn 

 in the preceding paragraph, with regard to the fun, is fo intirely applicable to the ftars, 

 that it will be very eafy to place this point alfo in its proper light. As I have affented to 

 the demonftration of opticians with regard to the brightnefs of the fun, when feen at the 



* See page 497. f Sec the definition of abfolute brightnefs, page 497. 



diftance 



