Dr Olbers on the Transparency of Space. 149 



is 6500 millions of times weaker than that of the full moon, or 

 732250 times weaker than that of the celestial vault in a clear 

 night, lightened by the full moon. Now, this last shade may 

 be considered as perfectly dark. 



We may therefore admit, that, with the degree of non-trans- 

 parency, which we have supposed to exist in space, the stars, 

 which are 30000 times farther from us than Sirius, do not con- 

 tribute to light the celestial vault. The ground of the sky 

 would therefore appear to us black, had not our own atmos- 

 phere, lightened only by the stars, itself a feeble lustre, which suf- 

 fices to colour this ground of a bluish tint. 



A circumstance which proves that the ground of the sky 

 would be entirely black, did we not see it through our atmos- 

 phere, which is lighted by the glimmer of the stars, exists in 

 what we observe regarding the planet Venus. The portion of 

 its disk, which is not lighted by the sun, is sometimes distin- 

 guished from the sky by a peculiar or phosphorescent light, 

 but never as being darker than the ground which it covers. 

 The same is also remarked in the planet Mars, when it is not 

 light all over. Those who have had occasion to observe the 

 starry sky on high mountains, have seen that it was dark, and 

 even absolutely black, although the greatest part of our atmos- 

 phere was still interposed. 



I do not know if I am deceived, but it has often seemed to 

 me, that, among the small stars, of the same luminous intensity 

 (the intensity is the lustre multiplied by the apparent magni- 

 tude), some had a mobile; and scintillating light, others a tran- 

 quil light. If this be not an illusion, I would be induced to 

 think that the former are smaller and nearer, the others larger and 

 more distant, in such a manner that the light of these latter, 

 weakened by the defect of transparency in space, has no longer 

 the density necessary for sparkling. 



The supposition that the light, independently of its divergence, 

 is weakened ijjij in coming from Sirius to us, is entirely arbi- 

 trary. My object was to demonstrate that this loss, and even 

 a still less at these enormous distances, was sufficient to render 

 the appearance of the heavens such as we observe it, although 

 the stars should yet exist in infinite number in space. It 

 is not without reflection that I have assigned this degree of 



