14i GETIA.RDI PiEGNERI FOCKENS 



« seconds : so Ihat bcsidcs the advanlage of discoverinjr the parallax of llie grcaler 

 « Star, if discernable, the difference of parall.ix of Ihat and of the lesser star ( being 

 « bolh within Ihe reach of a micrometer) may do cur work as well. For if Ihat of 

 « the greater star be discernable , but that of the lesser be either not discernable , or 

 « less discernable , their diflTerent distances from each otiier at diiTerent times of the 

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

 « of a competenl length, furnished wilh a micrometer, if carefuily preserved from being 

 <( disordered in the intervals of the observalions ; and discover at once , bolh , that there 

 <( is a parallax , and that the fixed stars are at different distances from us , wherein, 

 « that I be not mistaken, my meaning is not , that the inslrument or micrometer should 

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

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

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

 « from olher , in that position ( both being at once within the reach of the micromc« 

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

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

 « rallax of thcse two slars. " 



« T bis latter part of ttie Observation (of their different distances at different times) 

 « I suggest, as more ensiiy practicable though not so nice as tlie former. For it may 

 « be done, I tliink, without any further apparatus there than a good telescope, of or- 

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

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

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

 « the two soistices) be , one at the eastern , the other at the western azimuth (where- 

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

 « 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 another night 

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

 « discernably varied in a night or two. " 



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

 « yours, elc." 



§ ß- 



Huic subjungiraus RobertS| noimulla parum accurate raliocinantem , sed tarnen au- 



diendum ( ' ) : 



«The 



(*) Th'.l Trans. iGgi- XVIII, loi. Tilulus est: Conceming the dislance of the ßxcd Stars. Bj 

 the Honourtible Francis Roberts , Esij. S. R. S. 



