BIBLIOGRAPHY OF IIERSCHEl's WRITINGS. 507 



Herschel, W.: Synopsis of the Writings or— Contiimcd. 

 A. B. Yol. i\ 



The original eccentricity of the nebulous matter of a nebula may be 

 the physical cause of the rotatory motion of celestial bodies. 



1811 101 320 The nebulous star in Orion is lictitii>us. 



321 Two out of three nebulous stars in Orhm have lost their former nebu- 



lous ai>pearance. 

 Their fictitious appearance was owing to a dispersion of their light 

 in passing through nebulous matter. 



322 The faintest appearance of the nebulosity in Orion is perhaps not 



further from us than the stars of the third or second magnitude; 

 the brightest part is therefore probably not so far. 



323 In thirty-seven years the nebulosity of this nebula has undergone 



great changes, and much greater since the time of Huygiiexs. 



324 Nebnlte are not permanent celestial bodies. 



325 Additional proof of the opacity of the nebulous matter. 



325 Very distant nebulosities which cannot be seen in a state of diffusion 

 may become visible when condensed into separate uebuhe. 



327 Conversion of planetary into bright stellar nebuhe, into stars with 

 burs, or stars with haziness. 



329 Conversion of more distant ones into faint stellar nebula>, into stars 



with burs or with f:iint chevelure. 

 When it is doubtful whether an object is a star or a nebula, it may 

 be verified by an increase of magnifying power. 



330 When the object is very like a star it becomes difficult to ascertain 



whether it is a star or a nebula. 

 When we cannot ascertain whether the doubtful object is a star or a 

 nebula, of which several instances are given, the similitude is as 

 great as any we can expect ; for were it greater there could be no 

 doubt. 

 336 Postscript. 



[Dated] Slough, near Windsor, May 26, 1811. 



William Herschel. 

 336 Plates IV & V; 42 figures of nebuhe. 



1812 10*2 115 Observatiovs of a Comet, tvith remarks on the Confitriiction of its Different 



Farts. By William Herschel, LL. D., F. R. S. Read December 

 19, 1811. [This was Comet 1811, I.] 

 115 I have examined all the parts of the late comet with a scrutinizing 

 attention by telescopes of every degree of requisite light, distinct- 

 ness, and power. I have made so many observations that I shall 

 only give a selection of such as were made imder the most favor- 

 able circumstances. 



115 TJw Planetary Bodij in the Head of the Comet. 



Where with the naked eye I saw a luminous appearance not unlike 

 a star, with a telescope I found that this spot, which some might 

 call a nucleus, was only the head of the comet. 



116 Within its densest part was an extremely small bright point entirely 



distinct from the surrounding glare. Its contour was certainly not 

 otherwise than round, yet I could but very seldom perceive it de- 

 finedly to be so. 



116 I examined this point with various magnifiers on a 10-feet telescope. 



With 169 it was about 1".39 in diameter. 



117 With 600 it was between 1".06 and 0".68. The sealing-wax globules 



were viewed the morning after the observation as well as the morn- 

 ing before. [See Phil. Trans., 1808, p. 145.] 



