Dr Olbers on the Transparency of Space. 14«5 



But because the celestial vault has not, in all its points, the 

 lustre of the sun, must we reject the infinity of the stellar sys- 

 tem ? Must we restrict this system to a confined portion of li- 

 mitless space ? By no means. In the reasoning, by means of 

 which we arrived at the inference of the infinite number of the 

 stars, we have supposed that space was absolutely transparent, 

 or that the light composed of parallel rays was not impaired, as 

 it removed to a distance from the bodies from which it ema- 

 nated. Now, not only is this absolute transparency of space not 

 demonstrated, but, moreover, it is altogether improbable. What 

 though the planets, bodies possessed of great density, experience 

 no sensible resistance in their courses, there is nothing that can 

 oblige us to consider the space in which they move as perfectly 

 void. What may be presumed on the subject of cornets and 

 their tails, would rather tend to make us suppose the existence 

 of something material in the regions which they traverse. The 

 very matter of the tails of comets, which gradually dissipates, 

 and that of tlie zodiacal light, necessarily have their abode in 

 this space ; and, besides, supposing it absolutely void, the rays 

 of light, in crossing, might and must intercept each other. This 

 latter point may not only be demonstrated a priori^ by the hy-* 

 potheses of Newton and Huygens, regarding the nature of 

 light, but may also be experimentally confirmed by the compa- 

 rison of the telescopes of Cassegrain and Gregory, and the re- 

 lative density before and behind the focus of a spherical mir- 

 ror *. 



Space is not, therefore, absolutely transparent. But the 

 slightest defect in its transparency is sufficient to annihilate that 

 consequence of the infinite number of the fixed stars, so con- 

 trary to observation, namely, that the whole heaven should 



• Philosophical Transactions for 1813 and 1814. In the calculation of the re- 

 lative density of the light before and behind the focus of a concave mirror, Captain 

 Kater appears not to have reflected, that the focus cannot be considered as a phy- 

 sical point, but that it is only the place of the image of the sun, or of the flame of 

 a candle. This consideration ought to introduce some corrections into the calcu- 

 lations, but it does not affect the result that the light undergoes a loss in passing 

 through the focus. It would be desirable that these interesting experiments, 

 which perhaps might be directed in a manner better adapted to the object in view, 

 were repeated with great care. 



APEIL^-jyLY 1826. K 



