214 ^AUSE OF. THE CHANGES OF RELATIVE 



Theery of the motions, as well as the relative diftances of the three bodies are 

 ftws^^ " * determined, an hypothefis which admits the exiftence of fuch 

 motions and fituations, may refolve the phenomena of a double 

 flar, but cannot have any pretenfion to probability. 



The compafs of this Paper will not allow me to give the ob- 

 fervations of my double ftars at full length ; I fhall therefore, 

 in the examination of every one of them, only ftate thofe parti- 

 culars which will be required for the purpofe of inveftigating 

 .the caufe of the changes that have taken place, either in the 

 diflance, or angle of pofition, of the two flars of which thd 

 double flar iscompofed. 



As the arguments in the cafe of moft of thefe flars will be 

 nearly the fame, it may be expelled, that the firft two or three 

 which are to be examined will take up a confiderable fpace, 

 and the number of doable ftars, in which I have already afcer- 

 tained a change, amounting (o more than fifty, it will not be 

 poffible to give thera all in one paper ; I ftiali therefore confine 

 the prefent one to a moderate length, and leave it open for a 

 continuation at a future opportunity. 



a Geminorunu 

 Obfervatlonsand From my earlieft obfervations on the diflance of the two flars 

 inferences re- which make up the double flar in the head of Caftor, given in 

 ch^angef of re- *-^® ^^^ ^^ ^y catalogues of double flars, we find, that about 23 

 lative fituation years and a half ago, they were nearly two diameters of the 

 iyneareacT°'"^^!:se ^ar afunder. Thefe obfervations have been regularly 

 •ther. continued, from the year 1778 to the prefent time, and no al- 



teration in the diflance has been perceived : the flars are now 

 flill nearly two diameters of the large one afunder. 



It will be neceflary to enter a little into the pra6licability of 

 afcertaining diftances by a method of eflimation apparently fo 

 little capable of precifion. From a number of obfervations and 

 Experiments I have made on the fubjefl, it is certain that the 

 apparent diameter of a flar, in a refle6ling telefcope, depends 

 chiefly upon the four following circumflances : the aperture of 

 the mirror with refpeft to its focal length ; the difiin61nefs of the 

 mirror; the magnifying power ; and the flate of the almofpher* 

 at the time ®f obfervation. By a contraction of the aperture 

 we can increafe the apparent diameter of a flar,, fo as to maki 

 it refemble a fmall planetary difk. If diflindnefs ftiould 

 wanting, it is evident that the image of objeCls will not be ftiai 



an( 



