606 BIBLIOGRAPHY OF HEKSCHEL^S WRITINGS. 



Herschel, W.: Stnopsis of the "Writings of— Continnod. 



A. D. Vol. p. 



I have two mirrors for the 20-foot always ready, and my observations 

 are never interrupted by accidents, which often happen to large 

 mirrors, such as greatly injure their polish. 



1815 105 296 In these delicate observations no double eye-glass should be used, a? 

 we have a waste of light at four surfaces instead of two. 

 297 The hole through which the rays pass to the eye should be much 

 larger than the diameter of the optic pencils and considerably 

 nearer the glass than their focus, for the eye ought on no account 

 to touch the eye-piece. 

 297 With regard to the eye-glasses when merely the object of saving 

 light is considered, I can say from experience that concaves have 

 greatly the advantages of convexes, and they give also a much 

 more distinct image. This fact I established by repeated expen 

 ments about 1776. For the cause of the superior brightness and 

 sharpness of the image which is given by concaves we must prob- 

 ably look to the circumstance of their not permitting the reflected 

 rays to come to a focus. Perhaps a certain mechanical efleet, in- 

 jurious to distinctness, takes place at the focal crossing of rays in 

 convex lenses. 



297 [Foot-note.] [An experiment to test this described. It was incon- 



clusive.] 



298 The satellites were discovered with a magnifying power of 157 



only. 



299 Magnifiers of 300, 460, 600, and 800 have been used, according to the 



conditions of the mirror, atmosi^here, etc. On particular occasions 

 1,200 was used, and 2,400, 3,600, and 7,200 have been used to scruti- 

 nize the closest neighborhood of the planet. The known satellites 

 began to be nebulous with these powers. 

 299 In the following observations the positions of the satellites have been 

 determined in 3 ways : Coarse estimations, aided by diagrams ; 

 more careful ones, aided by a wire in the focus of the eye-glass ; 

 and micrometer measures. 



301 Distances were more difficult to measure than angles of position. 



302 The following observations are given in the order of the time they 



were made. They contain everj'thing that relates to the two 

 large satellites and to the researches for detecting additional satel- 

 lites. 

 That such there are I have no doubt. 



303 After each observation is given an "identitication" which shows by 



computation the places of the known satellites. 



304 The configurations made at the time of observation are not given. 



They generally contained the planet, its satellites and some of the 

 neighboring stars, especially those that were in the path of the 

 jilanet. 



304 Observations of the satellites of the Georgian planet, accompanied hy a the- 

 oretical determination of their situation, wherehi/ their identity may be 

 ascertained. [From 1787, Jan. 11, to 1810, May 25.] 



322 [Foot-note. ] Telescopic vision in windy weather is generally very per- 

 fect. 



343 Investigation of several particulars deduced from the foregoing observations, 



with an exposition of the method by which they have been obtained. 



344 The place of the ascending node, the inclination of the orbits ar.d the retro- 



grade motions of the satellites determined. 



