Diffused Cosmic Matter. 325 



comet in its last appearance, will not gainsay me ; the reserve 

 I recommend will appear to them, I hope, quite natural. 



Diffused cosmic matter, not luminous of itself, and imperfectlt/ 

 diaphanous. — Herschel thinks that he has determined, by the 

 observations I am about to mention, that besides the diffused 

 matter, luminous of itself, of which we have spoken so much, 

 there exists in space another equally diffused, but not radiat- 

 ing, and imperfectly diaphanous. 



In March 1774, this celebrated astronomer perceived on the 

 north of the great and beautiful nebula of Orion, on both 

 sides of the celebrated nebulous star signalized by Mairan, 

 two other smaller stars surrounded in the same manner with 

 circular nebulosities. 



In the month of December 1810, the nebulosities of these 

 two small stars were dissipated. On the 19th January 1811, 

 no trace of them was to be seen, even with a telescope of 

 39 feet. With regard to the nebulosity of the principal star, 

 it had undergone no change save becoming very much weaker. 



Herschel believed that the three nebulosities in question 

 were not real. When a star is seen through a mist, it appears 

 to be in the centre of a luminous glory. This glory is com- 

 posed of a portion of the mist illuminated by the star. An 

 analogous cause produced, according to this illustrious astro- 

 nomer, the nebulosities observed in 1774 around the three 

 stars mentioned ; only, the ordinary mist was replaced by a 

 cosmic matter, nearer to us than the three stars, situated, 

 however, in the high regions of the firmament, and in imme- 

 diate connection with the great nebula of Orion. The matter 

 did not shine with its own light, since, at a certain distance 

 from the three stars, no trace of it was seen. It reflected 

 strongly towards our eye* the starry rays which traversed it, 

 under incidences very little removed from the perpendicular ; 

 it wanted that extreme diaphaneity which our fancy takes plea- 

 sure in conferring on gaseous matters situated in the celestial 

 spaces ; finally, it was by obeying a clustering power, which 

 all the nebulous matter of Huygens is subject to, that it ceased 

 in 1810 to interpose itself exactly between the two small stars 



