other. 



rOSlTION OP DOUBLE STARS. ' 2l5 



and well defined, and that they will confequently appear larger Obfervationsand 

 than they ought. The effect of magnifying power is, to occafion fpe^ing the 

 a relative increafe of the vacancy between two liars that are changes of re- 

 very near each other ; but the ratio of the increafe of the dif- j^ ftars «trcme- 

 tance is not proportional to that of the power, and fooner or ly near each 

 later comes to a maximum. The flate of the atmofphere is per-* 

 haps the raoft material of the four conditions, as we have it not 

 in our power to alter it. The effeds of moifture, damp air, and 

 hazinefs, (which have been related in a paper where the caufes . 

 that often prevent the proper adion of mirrors were dif- 

 cuffed,) (how the reafon why the apparent diftanceof a double 

 ilar fhould be affedled by a change in the atmofphere. The 

 alteration in the diameter of Ardurus, extending from the firft 

 to the laft of the ten images of that ftar, in the plate accompany- 

 ing the above-mentioned paper,* (hows a fufficient caufe for an 

 increafe of- the diftance of two flars, by a contraction of their 

 apparent difks. A (kilful obferver, however, will foon know 

 what ftate of the air is mod: proper for eftimations of thi;; kind. 

 I have occafionally feen the two flars of Caftor, from l{ to 2 

 and 2| diameters afunder ; but, in a regular fettled temperature 

 and clear air, their diftance was always the fame. The other 

 three caufes which affe6lthefe eftimations, are at our own dif- 

 pofal ; an inftance of this will be feen in the following trial. I 

 took ten different mirrors cf feven feet focal length, each hav- 

 ing an aperture of 6,3 inches, and being charged with an eye- 

 glafs which gave the telefcope a magnifying power of 460. 

 With thefe mirrors, one after another, the fame evening I 

 viewed the two ftars of our double fiar; and the refult was, 

 that with every one of them, the ftars were precifely at an equal 

 diftance from each other. Thefe mirrors were all fufficiently 

 good to ftiow minute double ftars well; and fuch a trial will 

 confequently furnilh us with a proper criterion, by which we 

 may afcertain the goodnefs of our telefcope, and the clearnefs 



• of the atmofphere required for thefe obfervations. To thofe 



• who have not been long in the habit of obferving double ftars, 

 it will be neceffary to mention, that, when firft feen, they will 

 appear nearer together than after a certain time; nor is it fo 

 foon as might be expeded, that we fee them at their greateft 



* Sec Phil. Tranf. for 1803, page 232, Plate III. 



diftance. 



