Penetration into Space hy Tele/copes. 501 



• < 



diftance of Saturn, provided the meaning of this word be kept to the intrmfic illumination 



of the pifture on the retina of an obferver, I can have no hefitatlon to allow that the fame 



will hold good with a ftar placed at any affignable diftance. But I muft repeat, that the 



arl 

 light we can receive from ftars is truly expreffed by -y, ; and that therefore their abfolute 



brightnefs mufl vary in the inverfe ratio of tlie fquares of their diflances. Hence I am 

 authorifcd to conclude, and obfervation abundantly confirms it, that ftars cannot be feen by 

 the naked eye, when they are more than feven or eight times farther from us than Sirius ; 

 and that they become, comparatively fpeaking, very foon invifibie with our beft inftruments. 

 It will be fhewn hereafter, that the vifibility of ftars depends on the penetrating power of 

 telefcopes, which, I muft repeat, falls indeed very fliort of (hewing ftars that are many 

 thoufands of times farther from us than Sirius ; much lefs can we ever hope to fee ftars 

 that are all but infinitely diftant. 



If now it be admitted that the exprcflions we have laid down are fuch as agree with well 

 known fa£ts, we may proceed to vifion at a diftance ; and firft with refpedl to the 

 naked eye. 



Here the power of penetrating into fpace, is not only confined by nature, but is more- 

 over occafionally limited by the failure in brightnefs of luminous objeiSls. Let us fee 

 whether aftronomical obfervations, affifted by mathematical reafoning, can give us fome 

 idea of the general extent of natural vifion. Among the reflc£ting luminous objedls, our 

 penetrating powers are fufficiently afcertained. From the moon we may ftep to Venu? 

 to Mercury, to Mars, to Jupiter, to Saturn, and laft of all to the Georgian planet. An 

 objeft feen by reflected light at a greater diftance than this, it has never been allowed us to 

 perceive ; and it is indeed much to be admired, that we (hould fee borrowed illumination 

 to the amazing diftance of more than 18 hundred millions of miles; efpecially when that 

 light, in coming from the fun to the planet, has to pafs through an equal fpace, before it 

 can be reflefted, whereby it muft be fo enfeebled as to be above 368 times lefs intenfe on 

 that planet than it is with us, and when probably not more than one-third part of that light 

 can be thrown back from its dilk *. 



The range of natural vifion with felf-luminous obje£ls, is incomparably more extended, 

 but lefs accurately to be afcertained. From our brighteft luminary, the fun, we pafs im- 

 mediately to very diftant objedVs ; for, Sirius, Arfturus, and the reft of the ftars of the firft 

 magnitude, are probably thofe that come next ; and what their diftance may be, it is well 

 known, can only be calculated imperfedtly from the doftrine of parallaxes, which places 

 the neareft of them at leaft 412530 times farther from us than the fun. 



* According to Mr. Bouguer, the furface of the moon abforbs about two-thirds of the light it receives 

 from the fun. See Traite d'Optique, page izj; 



4 In 



