BIBLIOGRAPHY OF HERSCHEL's WRITINGS 503 



Herschel, W.: Synopsis of the Wkitixgs of— ConlimuMl. 

 A. D. Vol. P. 



1798 88 74 The satollitt's Ijcconio rogiilarly invisihlo at ccrtaiu distances from 



the planet. 



76 The first satellite [ 77/««(r(] usually vanishes at 18". 

 The second satellite lOberonl usually A-anishes at iiO". 



In uncommon and beautiful nights the fust 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 diffused 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 very nearly the dimmest objects that can bo 

 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 [^Ohcron']. The interior satellit(! cannot bo much inferior in 

 brightness to these. 

 78 Periodical Revolutions of the JVew Satellites. 



[Dated] Slough, near Windsor, September 1, 1797. 



1799 89 1'21 A Fourth Catalof/ue of the Comparative Brightness of the Stais. By Wil- 



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

 121 Catalogue. 

 138 Xotes. 



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



1800 90 49 On the poicer of penciraiing into Space hi/ Telescopes; with a comparative 



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

 escopes of various Sizes and Constructions; illustrated by select observa- 

 tions. By WiixiAM Herschel, LL. D., F. R. S. Read November 

 21, 1799. 

 49 The i)ower of penetrating into space by telescopes is very different 

 from magnifying power, and ought to be considered separately. 



49 Lumiuoiis bodies as here defined arc such as throw out light, what- 



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

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

 it is necessary. 



50 The question wiU arise whether luminous bodies scatter light 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 i)hysical points into which we 

 may suppose 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 Liminous gases are 

 penetrable to the rays of light. 



50 [Foot-note.] An experiment given to i)rove Ibis. 



51 It follows therefore that every part of the sun's disc cannot appear 



equally luminous to a given observer on account of the unequal 

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



