BIBLIOGRAPHY OF iii:ksciiet7s writings. 561 



Herschel, W.: Synopsis of Tm.; WiMTiNfis OF— CoiiMimcd. 



A. D V<,1. P. 



1797 87 \\?,-l OJm-rvaUons of the chanfjeahle BrUihtncss of the SatrlUtcs of Jiqnlir, and 

 of the Variatioti in their apjuireiit Magnitudes ; with a Determination of 

 the Time of their rotator n Motions on their Axes. To which is added a 

 treasure of the Diameter of the Second Satellite, and an Estimate of the 

 eomparatire s/ce of all the Fonr. I!y Wii.i.ia.m I1f,i;schi:l, LL. D., 

 F. R. S. lii'iid Juno 1, 17n7. 



1797 87 ;532 I have, when oilier pursuits would i.ennil, .-ittciuled to every circura- 

 staiice that oonhl forward the discovery of the rotation of the 

 secondary plau(d-s. Since I have deternuned by observation that 

 Japetns rotates according to the law obeyed by our moon, it seems 

 natural to conclude that all (he secondary planets do the same; 

 consequently, a, few good observati(»ns that coincide with this 

 theory will go a good way towards contirniing it. 



333 I also desired to examine the nature and construction of the satellites. 



Here pheuomeua occnrred that may be thought remarkable and 

 perhaps contradictory. So far from attempting to lessen the force 

 of such animadversions, 1 shall be the first to point out difiSculties 

 in order that future observations may be made to resolve them. 



334 Observations : A remarkable Conjwnction of two satellites of Jupiter. II 



and HI cannot be separated with a power of 350 on a 7-foot retlector. 



334 Inlenseness of Li(jht and Color of the Satellites. I is of a very iuteuse, 



bright, white, and shining light. It is brighter than II or IV (not 

 larger). IV is inclining to red. It is n(>arly as bright as II. II is 

 of a dull ash color. Ill is very white. 



335 Brightness and diameter distinguished. 



335 Diameter of the second Satellite by entering on the Disc of the Planet. 

 339 The Brightness of the Satellites compared to the Belts and Disk of the 



Planet. 

 339-342 [Estimates of comparative magnitudes of I, II, III, IV.] 



342 Before we draw any conclusions from these observations, we ought to 



take notice of the many causes of deception, etc. The method of 

 comparing the brightness is not subject to so great errors as esti- 

 mating this in terms of an ideal standard. But the situation of the 

 ^ satellites with respect to the i^lauet introduces a new source of 



error. 



343 Objections to both low and high magnifiers. 



344 It appears that considerable changes take place in the brightness of 



the satellites, and also a change in their ai)parent magnitude. 



344 The fii'st fact proves that the satellites have a rotatory motion upon 



their axes of tlKs same duration with their revolutions about the 

 primary planet. The second either shows that the bodies of the 

 satellites are not spherical, or that some parts of the discs reliect 

 hardly anj' light. 



345 Discussion of tli(Mjbser\ati()us t(j show that the satellites revolve on 



their axes in the same ( ime that they revol vi- about the planet. The 

 observations extend from 1794, July 19, to 1795, November :>. 



3^6 Table of thej^ositions of the four satclliirs at the lime of the observations. 



347 Method of reduction. 



349 Summary of the observat ions relatiug to the color of the satellites. 



I is white, but sometimes more intensely so than others. 



II is white, bluish, and ash.-colored. 



350 III is always white, but the color is of diflerent intensity in difierent 



situations. 



S. :\iis. ;;i .30 



