BIBLIOGRAPHY OF HEKSCHEL's WRITINGS. oGl 



Herschel, W.: Synopsis of tiik Wiutings of — Continiu-d. 



A.D Vol. P. 



1797 87 332 Observations of the cliaiKjcablc Brii/IiinesH of the SdtetlUea of Jupitir, and 

 of the Variation in their apparent ilagnitndef< ; with a Determination of 

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

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

 comparative size of all the Four. l)y ^^'ILI.IA^I IlKKsciiKr,, I.L. D., 

 F. R. S. R(-';ul June 1, 1797. 



1797 87 332 I have, when othci- pursuits would permit, iittendpd to i-very circuin- 

 .stiince tluit could forward the discovery of the rotation of tho 

 secondary planets. Since I have determined by observation that 

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

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

 couseciuently, a few good observations that coincide with this 

 theory will go a good way towards conlirming it. 



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



Here phenomena occurred that may be thought remarkable and 

 perhaps contradictory. So far from attempting to lessen the forco 

 of such animadversions, I shall be the hrst to point out difticulties 

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



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



and III cannot be separated with a power of 330 on a 7-foot reHector. 



334 Intenseness of Light and Color of the Satellites. 1 is of a very intense, 



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

 larger). IV is inclining to red. It is nearly as bright as II. II is 

 of a dull ash color. Ill is very Avliite. 



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 wo 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 planet introduces a new source of 

 error. 



343 Objections to both low and high magniliers. 



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



the satellites, and also a change in their apparent magnitude. 



344 The tirst fact proves that the satellites have a rotatory motion upon 



their axes of the same duration with their revolutions about the 

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

 satellites arc not spherical, or that some parts of the discs reilect 

 hardly any light. 



345 Discussion of the observations to show that the satellites revolve on 



their axes iu the same time that they revolve about the planet. The 

 observations exteml from 1794, .July 19, to 179(3, November 3. 



34G Table of the positions of the four satellites at the time of the observations. 



347 Method of reduction. 



349 Summary of the observations relating to the color of the sat.dlites. 



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 dilTerent intensity m dillereut 



situations. 

 S. :\Iis. .",1 30 



