PORTION OF THE COSMOS. COSMICAL SPACE. 41 



undulations propagate light, because resistance implies non- 

 penetration of what is solid. The observations require for 

 the explanation of the diminished period of revolution, (the 

 diminished major axis of the ellipse), a tangential force ; 

 and this is supplied in the most direct manner by the as- 

 sumption of a resisting fluid. ( 83 ) The greatest effect shows 

 itself in the twenty-five days next before the passage of the 

 comet through its perihelion, and in the twenty-five days 

 which follow the passage. Thus the value of the constants 

 is somewhat different, because near the sun, the so rare, 

 but yet gravitating, strata of the resisting fluid are denser. 

 Olbers ( s4 ) was of opinion that the fluid could not be in 

 repose, but must rotate from right to left round the sun ; 

 and, therefore, the resistance to retrograde comets, like 

 Halley's, must be quite different from the resistance to a 

 comet whose motion is direct, as Encke's. The calcu- 

 lation of perturbations in comets of long period, and 

 the differences of, masses and magnitudes of the comets, 

 complicate the results, and mask what may be due to par- 

 ticular causes. 



The nebulous or vaporous matter which forms the ring 

 of zodiacal light, may be, perhaps, as Sir John Herschel ex- 

 presses it, only the denser part of the comet-resisting medium 

 itself. ( 85 ) Even supposing it were already proved that all 

 nebulae are only imperfectly- seen crowded clusters of stars, 

 there yet remains the fact, that innumerable comets, by 

 the dissolution of their tails of more than fifty millions of 

 miles in length, fill space with a material* substance. 

 Arago has ingeniously shown from optical considerations, ( 86 ) 

 that the variable stars which in their periodical phases 

 always show white light, without any trace of colour, might 



