ON THE CONSTRUCTION OF THE HEAVENS. 79 



orbits by the fun, yet their mutual fubordinate fyftem is fueh> 

 that they perform fecondary monthly revolutions round a centre 

 without a body placed in it. The fame indeed, though under 

 very narrow limits, may be faid of the fun and each planet 

 itfelf. 



That no infulated ftars, of nearly an equal lize and diftance, If inflated ftars 

 can appear double to us, may be proved thus. Let Arfturus and ^ f^*?J° r * 

 Lyra be the ftars: thefe, by the rule of infulation, which we Arfturus and 

 nrnft now fuppofe can only take place when their diftance from L y ra > the y can ~ . 

 each other is not lefs than that of Sirius from us, if very accu- mo tenefs from 

 rately placed, would be feen under an angle of 60 degrees from the eartft a ppe ar 

 each other, They really are at about 59°. Now, in order to 

 make thefe ftars appear to us near enough to come under the 

 denomination of a double ftar of the firft clafs, we fhould re- 

 move the earth from them at leaft 41253 times farther than 

 Sirius is from us. But the fpace-penetrating power of a 7-feet 

 reflector, by which my obfervations on double ftars have been 

 made, cannot intitle us to fee ftars at fuch an immenfe diftance ; 

 for, even the 40-feet telefcope, as has been (hewn *, can only 

 reach ftars of the 1342d magnitude. It follows, therefore, that becaufe they 



thefe ftars could not remain vifible in a 7-feet refkaor, if they . wo ?i d L1 b " ome 



r r i 1 , • ^ t r , wvifible before 



were io tar removed as to make their angular diftance lets than they were re- 



about 24J minutes; nor could even the 40-feet telefcope, un- mote enough, 

 der the fame'circumftances of removal, fhew them, unlefs they 

 were to be feen at leaft 2| minutes afunder. Moreover, this 

 calculation is made on a fuppofition that the ftars of which a, 

 double ftar is compofed, might be as fmall as any that can 

 poffibly be perceived ; but if, on the contrary, they mould 

 ftill appear of a considerable lize, it will then be fo much the 

 more evident that fuch ftars cannot have any great real dif«. 

 tance, and that, confequently, infulated ftars cannot appear 

 double, if they are fituated at equal diftances from us. If, 

 however, their arrangement (hould be fuch as has been men- 

 tioned before, then, one of them being far behind the other, 

 an apparent double ftar may certainly be produced ; but here 

 the appearance of proximity would be deceptive ; and the ob- 

 ject fo circumftanced could not be claifed in the lift of binary 

 fyftems. However, as we muft grant, that in particular fitu-It is highly im- 

 ations ftars apparently double may be compofed of fuch as are^ rs *f*'' L that tikZ 



fhould confiftof 



* See Phil. Tranf. for 1800, Parti, page 83. ftars confide- 



infulated, rab) y afunder » 



