SUCCESSIVE PROPAGATION OF LIGHT. 143 



most crowded groups of small stars with which the heavens 

 are adorned. Herschel, indeed, ascribes to the attractive 

 force of these marginal groups ( 117 ), the starless open- 

 ings themselves ; of which he says, in his finely ani- 

 mated style, " they are parts of our sidereal stratum 

 which have already suffered great devastation from time." 

 If we consider the telescopic stars, situated one behind 

 another, as forming a canopy of stars covering the 

 whole apparent celestial vault, we may, I think, regard the 

 starless portions of Scorpio and Ophiucus as tubes through 

 which we look into the remote regions of space. The strata 

 which form the canopy are there interrupted ; other still 

 remoter stars may indeed lie beyond, but our instruments 

 cannot reach them. The ancients had also been led, by the 

 apparition of igneous meteors, to the idea of rents or chasms 

 in the canopy of the skies ; but the chasms were supposed 

 to be only transitory, and, instead of being dark, to be 

 bright and fiery, from affording a glimpse of the burning 

 ether beyond ( 118 ) . Derham, and even Huygens, appeared not 

 indisposed to explain, in a somewhat similar manner, the 

 tranquil light of the nebulse ( 119 ). 



When we compare the stars of the first magnitude, 

 which on an average are certainly the nearest to 

 us, with the non-nebulous telescopic stars, and the ne- 

 bulous stars with unresolvable nebulse (for example, with 

 the nebula in Andromeda, or even with the so-called 

 planetary nebulse), and when we thus enter on the consi- 

 deration of distances so diverse in the boundless regions 

 of space, there presses itself on our notice a fact, which 

 governs the relation of the phenomena as perceived by us, 

 to the realities which are their actual basis, viz. the sttcces- 

 sive propagation of light. The velocity of this propagation, 



VOT.. T M 



