BIBLIOGRAPHY OF nEKSCIIEL's WRITINGS. 527 



Herschel, W.: Synopsis of the Writings oi" — Continuod. 



A.B. Xol. r. 



all its planets and comots, may also have a motion toward some 

 partievdar point of the heavens. If this snrmiscs shonld have any 

 fonndatiou it will show itself in a scries of some years in a kind 

 of systematical parallax or change dnc to the motion of the whole 

 solar system. 



1782 72 101 Theory of the annnal parallax of donble stars ; General Postnlata : 

 1. Let the stars be supposed, one with another, to bo about the size 

 of the snn. 

 104 2. Let the dilference of their apparent magnitudes be owing to their 

 dilferent distances, so that a star of the 2d, 3d, 4th magnitude is 2, 

 3, 4 times as far off as one of the first. 



104 [Foot-note.] This is rather the order into which stars ought to be 



divided than that into which they are. And perhaps we ought to 

 make an allowance for some loss which may hajipeu to the light of 

 veiy remote stars in its passage through immense tracts of space 

 most probably not quite destitute of some very subtle medium. 

 I have usually found very small telescopic stars to be red, or inclined 

 to red. 



105 Case of the parallax of stars in the ecliptic. 



106 The following general expression for the parallax of two stars : 



Let P express the total parallax of a fixed star of the first magnitude; 

 M the magnitude of the largest of the two stars; m the magnitude 

 of the smallest, audi? the partial parallax to be observed by the 



change in the distance of a double star ; then will j) = — ^|— • ^ 

 and J) being found by observation will give us P = -^ ^^j. 



107 Cases where the stars are not in the ecliptic. 



1782 72 112 Catalogue of DoiiMe Stars. By Mr. Heeschel, F. R. S., communicated 

 by Dr. Watson, juu. Read January 10, 1782. 

 112 Introductory remarks. The catalogue contains, I. The names of the 

 stars, etc. II. Their comparative size. III. Their colors. IV. Theij 

 distances, Avhich are given several ditferent ways. Those estimated 

 by the diameter can hardly be liable to an error of so much as 0".25. 

 Those measured by the micrometer may have errors from 1" to 2". 

 A new micrometer [Lamp-Micrometer, P. T., 1782, p. 1(53], made 

 within a few months, gives measures which can be relied on to 0".l 

 when a mean of three measures is taken. V. The angl(>s of position ; 

 when measured they can be relied on to 2° or 3=". "VI. The dates of 



discovery. 



115 Catalogueof Double Stars; First Class [24 stars]. 



120 h Draconis : " It is in vain to look for them if every circumstance is 

 pot favorable. The observer as well as the instrument must have 

 been long enough out in the open air to acquire the same tempera- 

 ture. In very cold weather an hour at least will be required ; but 

 in a moderate temperature half an hour will be sufficient." 



122 Second Class of Double Stars I'.^d stars]. 



128 f Sagitt(B : the small star brighter with 460 than with 227 or 278. 



129 Third Class of Double Stars [46 stars]. 



131 7 DdpUni: "I suspect a motion in one of these stars. I thought it 



best not to join other observations" to those of 1779. 

 136 Fourth Class of Double Stars [44 stars]. 

 142 Fifth Class of Double Stars [51 stars]. 



