Vlanetary Nebulas, 323 



der to be in every respect comparable, observations on objects 

 of such small brilliancy, and so ill defined, ought to be made 

 at all the different periods with glas^ses of the same power ; 

 but this condition had not been attended to. Herschel, on 

 the contrary, strictly conformed to it. His telescope, in 1811, 

 in no respect differed from the instrument of 1783. This 

 gave him the confidence to say : I have proved these changes, 

 {Phil. Trans. y 1811, p. 324). The proof ^\di not appear so in- 

 disputable as to prevent the son of Sir William from recently 

 ranking himself among the sceptics. .John Herschel's beautiful 

 memoir is too much out of the plan I have chalked out for 

 myself, to permit me to analyze it in this place. 



Planetary Nebulce. Is it true thatf in order to explain the 

 uniform luminosity of their discs, it is indispensably necessary 

 to suppose that the diffused matter is opaque after it reaches a 

 certain degree of concentration ? — Herschel applied the above 

 name to nebulae which resemble the planets of our system 

 in form. They are circular or slightly elliptical ; some have 

 their contours distinctly defined ; others appear surrounded by 

 a slight nebulosity ; their light is equally bright over the whole 

 extent of the disc. Among the planetary nebulae discovered 

 by Herschel, I find some of ten, fifteen, thirty, and even sixty 

 seconds in diameter. 



Herschel regarded the physical constitution of planetary 

 nebulae as very problematical. His fertile imagination could 

 furnish him with nothing very plausible or satisfactory on this 

 subject. These bodies could not be likened to the globular 

 nebulae composed of stars, without explaining why their light 

 did not present any increase of intensity towards the centre. 

 To transform the planetary nebulae into stars, properly so 

 called, was to disregard all analogy ; it was to create stars 

 with actual diameters thirteen thousand times greater than the 

 diameter of the sun (diameters of 4600 millions of leagues), 

 and to ascribe to stars a kind of dull light which no star has 

 hitherto exhibited. 



After much hesitation, Herschel decided on considering the 

 planetary nebulae as agglomerations, already very much con- 

 densed, of the diffused matter. This assimilation, it cannot 

 be disguised, demands a hypothesis which appears not v^ry 



