556 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Herschcl, XT.: Sv^'orsis or xiii: Writings of— Continued. 



A. D. Vol. P. 



1795 to 71 It seems, therefore, upon tlie whole not improbable that in many casea 

 stars are united in such close systems as not to leave much room for 

 the orbits of planets or comets ; and that consequently many stars, 

 unless we would make them mere useless brilliant points, may them- 

 selves be lucid planets, perhaps unattended by satellites. 



71 Postscript. The following observations of the sun are added. They 

 are decisive in regard to one of the conditions of the lucid matter 

 of the sun. 1794, Nov. 20. Tbo sun is mottled everywhere, equally 

 at poles and equator. This is owing to inequalities in its surface. 



71 The hicid substance of the sun is neither a liquid nor an clastic lluid, 

 as is evident from its not instantly lilliug up the cavities of the 

 spots, etc. It exists, therefore, in the manner of lucid clouds swim- 

 ■ ming in the transparent atmosphere of the sun; or, rather, of lu- 

 minous decompositions taking place within that atmosphere. 



1795 85 ^V7 Description of a fortij-fooirejlect'mj telescope. By William Herschel, 

 LL.D., F. E. S. Read Jane 11, 1795. 



347 When I resided at Bath I had long been acquainted with the theory 



of optics and mechanics and wanted only that experience so neces- 

 sary in the practical part of these sciences. This I acquired by 

 degrees at that place, where in my leisure hours by way of amusj- 

 ment I made several 2-foot, 5-foot, 7-foot, 10-foot and 20-foot New- 

 tonian telescDpes, besides others of the Gregorian form of 8, 12, 18 

 inches and 2, ?>, 5 and 10 feet focal length. 

 34S In this way I made not less than 200 7-foot, 150 more 10-foot, and 

 about 80 20-foot mirrors, not. to mention the Gr<gorian telescopes. 



348 The number of stands I invented for these telescopes it would not bo 



easy to assign. My Newtonian stand was contrived about 1778. 



348 In 178 L I began to construct a 30- foot aerial rellector, and having made 



a stand for it I cast the mirror 3G inches in diameter, which was 

 cracked in cooling. I cast it a second time, and the furnace which 

 I had built in ray house broke. Soon after, the Georgian Planet 

 was discovered [and observations on this interrupted the making 

 of new telescopes. ] 



349 In 1783 I finished a very good 20-foot reflector, and in 1785 I began to 



construct the 40- foot. 



349 In the whole of the apparatus none but common workmen were em- 



ployed, for I made drawings of every part of it, and directed every 

 jierson's labor, though sometimes there were not less thau 40 em- 

 ployed at th(» same time. There was no interruption except my re- 

 moval from Clay Hall to Slough. 



350 The 40-foot speculum was x)ut into the tube and first used 1787, Feb. 



19. The first mirror being too thin, etc., could not receive a good 

 figure, and a second was cast 1783, Jan. 20, which cracked in cool- 

 ing. It was recast Feb. 10, and on Oct. 24 it had a good figure and 

 I observed Saturn with it ; but not being satisfied I worked upon it 

 till Aug. 27, 1789. Aug. 28, 1789, I discovered a sixth satellite of 

 Saturn. I date the finishing of the telescope from that time. 



350 Description of the telescope. See Plates XXIV to XLII. 



350-365 The foundation and stand. 



365 The tube : it is 39 feet 4 inches loug, 4 feet 10 inches diameter, made 

 of iron. 



377 Motions of the telescope. 



