564 PHILOSOPHICAL TRANSACTIONS. [aNNO I693. 



may chance to be nearer to us than what we try at first. And stars of this size 

 may be observed by a moderate telescope, even in the day-time, especially when 

 we know just where to look for them. 



The manner of observation I conceive may be thus ; having first made choice 

 of the star we mean to observe, and having then considered where such star is 

 to be seen in its greatest east or west azimuth ; it may be then convenient to 

 fix very firm and steadily, on some tower, steeple, or other high edifice, in a 

 convenient situation, a good telescopic object-glass, in such position as may be 

 proper for viewing that star ; and at a due distance from it, near the ground, 

 build some little stonewall, or such like place, on which to fix the eye-glass 

 so as to answer that object-glass ; and having so adjusted it, as through both to 

 see that star in its desired station, which may best be done while the star is 

 to be seen by night in such situation, near the time of one of the solstices ; 

 let it be there fixed so firmly, as not to be disturbed, and the place so secured* 

 as that none may come to disorder it, and care be taken to defend both the 

 glasses, so as not to be endangered by wind and weather. This glass being once 

 fixed, and a micrometer fitted to it, so as to have its threads perpendicular to 

 the horizon, to avoid any inconvenience which might arise from diversity of 

 refraction, if there be any, the star may then be viewed from time to time, 

 for the following year or longer, to see if any change of azimuth can be 

 observed. 



The reason why I chiefly recommend, as a convenient star for this purpose, 

 the shoulder of the Lesser Bear, is that there is adhering to it a very small star, 

 which the Arabs call Alcor, (of which they have a proverbial saying, when they 

 would describe a sharp-sighted man, that " he can discern the rider on the 

 middle horse of the Wain ;" and of one who pretends to see small things but 

 overlooks much greater, that " he sees Alcor, but not the full moon,") and 

 which Hevelius finds to be distant from it about 9' and 5 or 10'': so that, be- 

 sides the advantage of discovering the parallax of the greater star, if discerni- 

 ble, the difference of parallax of that and of the lesser star, being both within 

 the reach of a micrometer, may do the business as well : for if that of the 

 greater star be discernible, but that of the lesser be either not, or less descerni- 

 ble ; their different distances from each other, at different times of the year, 

 may perhaps, without farther apparatus, be discerned by a good telescope of a 

 competent length, furnished with a micrometer, if carefully preserved from 

 being disordered in the intervals of the observations ; and discover at once, 

 both that there is a parallax, and that the fixed stars are at different distances 

 from us. And here my meaning is not that the instrument or micrometer 

 should be removed for the observing of the lesser star ; but that, when the 



