144 GERARDI REGNERI FOCKENS 



« scconds : so that besides tlie advantage of discovering the pnrallax of Ihe g^-ealer 

 « Star, if discernable, the difference of parallax of ihat and of llie lesser star ( being 

 « both within the rench of a micrometer) may do our werk as well. For if Ihat of 

 « the greater Star be discernable, bul that of the lesser be either not discernable, or 

 « less discernable , their difTerent distanees from each other at diiferent times of the 

 « year may , perhaps ( withont farther apparatus ) , be discerned by a good tclescope 

 « of a eorapefent lenglh , fiirnished with a micrometer , if carefully preserved from being 

 « disordered in the inlervals of the observalions ; and discover at once, both, that there 

 « is a parallax, and that the fixed stars arc at different distanees from us, wherein, 

 « that I be not mistaken, my meaning is not, that the instriiment or micrometer shoiild 

 « be removed for the observing of the lesser star ; but that ( when te azimuth of the 

 « greater star is taken ) by a micrometer ( consisting of divers fine threads parallel and 

 « transverse ) may ( at the same time ) be observed the distance of the two stars , each 

 « from other, in that position (both being at once wilhin the reach of the micromc- 

 « ter; ) which distance (the instrument remaining unmoved) if it be found (at different 

 « times of the year) not to be the same; this will prove, that there is a different pa- 

 « rallax of Ihese two stars. " 



« This lalter pari of tlie Observation ( of tlieir different distanees at different times) 

 « I suggest, as more easily pvacticable though not so nice as tlie forraer. For it may 

 « be done, I tliink, witliout any fuither apparatus there than a göod lelescope , of or- 

 « dinary form , furnished with a micrometer , ( this being carefully kept unvaried du- 

 « rin" the interval of these observalions.) And if this part only of the Observation 

 « (without the other) be pursued ; it matlers not thotigh the two observalions (near 

 « the two soisticcs) bc , one at the eastern , tlie other at the western azimuth (where- 

 « by both may be taken m the night-time , ) : for the distance must ( at both azimuth s ) 

 « be the same, if after observing the azimuth of the greater star it be necessary to move 

 « the micrometer for measuring its distance from Alcor that may be done auother night 

 « ( and it is not necessary to be done at one Observation ) for that distance cannot be 

 « discernably varied in a night or two. " 



« I shall give you no farther trouble at present , but subscribe my seif, Sir ! 

 « yours, elc. " 



§ 6. 



Huic subjungimus Roberts, nounuUa parum accurate ratiocinantem , sed tarnen au- 



diendum (•) : 



« The 



(*) P/i7 Tram. 1C94. XVIII, loi. Titulus est: Concerning the distance of the ßxcd sturs. By 

 ihe HonourabU Francis Bober ts , Esq. S, R. S. 



