BIBLIOGRAPHY OF HERSCnEL's WRITINGS. 083 



Herscliel, W.: Syxopsis of the Wiutixos of— Conlimu'd. 



A. B. Tol. r. 



1803 93 341 There are three motions to he cousidercd of the hodies O, a, x- :i i"0- 

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



341 Single Motions. If a and are at rest, a motion of x lu^iy ^^c assumed 



so as ])erfectly to exidain any change of distance or of angle of 

 position of the two stars. 



342 The etJect of a mot ion of a or of explained. 



343 Double motions. 



344 Motions of the three hodies. 



345 [Observations] case of a Geniino>-i/>n. 



The distance has remained about constant for 23^ years. 



345 In a rellector 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 "wo can increase the apparent diameter; 

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

 ing power increases the distance apart of two stars, hut 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. 

 34G 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 Geminonm. 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 0".34. 



348 The angle of position of these stars was in 177U, 32- 47' north-preced- 



ing; it is now only 1Q° 53'. In 23* years it has diminished 2F ..4 ; 

 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 P from 2 measures; in the worst circumstances the error 



will not be 3^^. , ■ x i i 



850 The cause of this change must be examined. A revolving .star woul.l 

 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]. 



^'S^ AH are inconsistent with my observations. 



S The proper motion of the sun must be admitted m 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 



give the preference [to the hypothesis of a revolution of the stais 

 themselves]. 



