BIBLIOGRAPHY OF HERSCIIEL's WRITINGS. 583 



Herschel, W.; Synopsis of the Writings of— Coutimicd. 



A. B. Vol. r. 



1803 93 341 There are three motions to be considered of the bodies O, a, x\ ^ mo- 

 tion of one, of two, or of all three. 



341 Single Motions. If a and are at rest, a motion of x may he assumed 



so as perfectly to explain any change of distance or of angle of 

 position of the two stars. 



342 The effect of a motion of a or of O explained. 



343 Double motions. 



344 Motions of the three bodies. 



345 [Observations] case of a Geminorum. 



The distance has remained about constant for 23^ years. 



345 In a reflector the apparent diameter of a star depends upon: the 



aperture with respect to the focal length ; the distinctness of the 

 mirror; the magnifying power; the state of the atmosphere. By 

 contracting the aperture we can increase the apparent diameter; 

 want of distinctness does the same thing; an increase of maguify- 

 ino- power increases the distance apart of two stars, but this in- 

 crease is not proportional to the increase of the power, and sooner 

 or later comes to a maximum; the state of the atmosphere is the 

 most material condition, as we cannot alter it. 



346 The other three causes are at our disposal. I took ten different mir- 



rors of 7 feet focus, 6.3 inches aperture, magnifying power 460. 

 With these, one after another, I viewed a Geminorum. With each 

 one the distance of the components was the same. 



346 When double stars are first seen they appear nearer together than 



later. 



347 I have known it to take up two or three months before the eye was 



sufficiently acquainted with the object to judge with the requisite 

 precision. 



347 The error of this method of estimating the distance of these two stars 



is not above G".34. 



348 The angle of position of these stars was in 1779, 32° 47' north-preced- 



ing; it is now only 10° 53'. In 23i years it has diminished 21° 54 ; 

 this change has been regular and gradual. 



348 Accuracy of the angles of position investigated by the deviation of 



separate measures from the mean. 



349 This micrometer, then, will give the position of a double star true to 



about 1° from 2 measures; in tho worst circumstances the error 



will not be 3°. , ■ ^ n 



350 The cause of this change must be examined. A revolving star would 



explain the alteration of the angle of position with no alteration of 

 the distance. As we have no precedent for, this it will be right to 

 examine whether the change cannot be accounted for by the proper 

 motions of the stars, or of the sun. 

 350 Single motions [examined]. 



355 All are inconsistent with my observations. 



356 The proper motion of the sun must be admitted in such a direction 



and of such a velocity as will satisfy the mean direction and velocity 



of the general proper motions of the stars. 

 356 Double Motion. This hypothesis is not maintainable. 

 359 Motion of the three Bodies. This hypothesis will explain the observa- 



tions but in so complex a way as to leave no doubt that we should 



g've the preference [to the hypothesis of a revolution of the stars 



themselves]. 



