BIBLIOGRAPHY OF HERSCHEL's WRITINGS 563 



Herschel, W.: Synopsis of the Writings of— Coutinued. 

 A. D. Vol. P. 



1798 88 74 The satellites become regularly invisible at certain distances from 



the planet. 



76 The first satellite ITitania'] usually vanishes at 18". 

 The second satellite lOberonl usually vanishes at 20". 



In uncommon and beautiful nights the first has once been seen at 

 13". 8 and the 2d at 17".:}. 



77 A dense atmosphere to the planet would account for this if it were 



not that the satellites are lost as well in the nearest half of their 

 orbits as in the farthest. A satellite cannot be obscured by an at- 

 mosphere that is behind it. 



77 The light of Jupiter and Saturn is diiiused for several minutes [o/ arc] 



all around them. Their satellites are bright, and having much 

 light to lose they comparatively lose but little. [ Uranus'\ is very- 

 faint ; its satellites are verj^ nearly the dimmest objects that can be 

 seen in the heavens, so that they cannot bear any considerable 

 diminution of their light without becoming invisible. 



78 The distances at which the satellites vanish will show their relative 



brightness. The first satellite [ Titania'] is rather brighter than the 

 second \^Oheron'\. The interior satellite cannot be much inferior in 

 brightness to these. 

 78 Periodical Rerolutions of the New Satellites. 



[Dated] Slough, near Windsor, Sei^tember 1, 1797. 



1799 89 121 A Fourth Catalogue of the Comparatire Brightness of the Stars. By Wil- 



liam Herschel, LL. D., F. R. S. Read February 21, 1799. 

 121 Catalogue. 

 138 Notes. 



[Dated] Slough near Windsor, .Jan. 28, 1799. 



1800 90 49 On the power of penetrating into Spare hij Teleseopes ; with a comparative 



Determination of the Extent of that Power in natural Vision, and in Tel- 

 escopes of tarious Sizes and Constructions ; 'illustrated by select observa- 

 tions. By William IlERSCnEL, LL. D., F. R. S. Read November 

 21, 1799. 

 49 The ]iower of penetrating into space by telescopes is very different 

 from magnifying power, and ought to be considered separately. 



49 Luminous bodies as here defined are such as throw out light, what- 



ever may be the cause of it, including those that shine by reOection 

 only, and we may distinguish the class of self-luminous bodies when 

 it is necessary. 



50 The question will arise whether luminous bodies scatter liglit in all 



directions equally ; but until we know more of the powers that emit 

 and reflect light we shall probably remain ignorant on this head. 



50 What I mean to say relates only to the physical points into which we 

 may suiipose the surfaces of luminous bodies to be divided. When 

 we consider their whole construction the question assumes another 

 form. 



50 We know from experience that light, flame, and luminous j^jases are 

 penetrable to the rays of light. 



50 [Foot-note.] An experiment given to prove this. 



51 It follows therefore that every part of the sun's disc canuo! appear 



equally luminous to a given observer on account of tins unequal 

 depth of its atmosphere. (See Phil. Trans., 1795, p. 40.) 



