BIBLIOGRAPHY OF IIERSCHEL's WRITINGS. 597 



Herschel, W.: Synopsis of the Wkitings of— Contimied. 

 A. D. Vol. r. 



Tlif original ect-oiitricity of Ihe nebulous iiiattor of a nebula may be 

 tlie physical cause of tbo rotatory motion of celestial bodies. 



1811 101 320 The nebulous star iu Orion is fictitious. 



3"21 Two out of three nebulous stars in Orion have lost their former nebu- 

 lous appearance. 

 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 jiart is therefore ])robably not so far. 



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



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



324 Nebuho are not permanent celestial bodies. 



325 Additional proof of tbo oi)acity of the nebulous matter, 



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

 may become visible when condensed into separate nebula;. 



327 Conversion of planetary into bright stellar nebuLe, into stars Avith 

 burs, or stars with haziness. 



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



with burs or with faint chevelure. 

 When it is doubtfnl 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 difTicult to ascertain 



whether it is a star or a nebula. 

 When wo cannot ascertain whether the doubtfnl object is a star or a 

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

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

 doubt. 

 336 Postscript. 



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



W^ILLIAM HeKSCIIEL. 



336 Plates IV «fe V ; 42 figures of nebuhe. 



1812 102 115 Obserrations of a Comet, with remarln on the Construction 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 refpiisite light, distinct- 

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

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

 able circumstances. 



115 The Flanctary Body 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 ]>oinf entirely 



distinct from the surrounding glare. Its contour was certainly not 

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

 fiuedly to be so. 



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



With 169 it was about 1".39 in dianu-ter. 



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 Fhil. Trans., 1808, p. 145.] 



