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THE POPULAR SCIENCE MONTHLY. 



in this special instance of the Pleiades they are not so much dazzled 

 by the brighter stars as guided by them to the stars in their neigh- 

 borhood, for, in fact, more than one half of the fourteen stars are of 

 a magnitude far below the commonly-accepted limit of vision for the 

 naked eye. We have learned to observe, to choose favorable condi- 

 tions, to know what is a really clear atmosphere; we know that small 

 stars in the vicinity of bright ones are far more readily descried in 

 twilight than in the depth of night, the brightness of the larger stars 

 in the latter case obscuring the smaller. Hipparchus errs in saying 

 that moonlight is a hinderance to such observation : keen eyes may, 

 with a bright full moon shining, count as many as fifteen stars in the 

 Pleiades. 



Another point of considerable interest we note in this instructive 

 example. The fact that the Postilion, the Alcor of the Arabians, was 

 taken to be the lost seventh star of the Pleiades, further shows that 

 Alcor, though a star of the fifth magnitude, and easily discernible, 

 had not been noted by previous astronomers, else it could never have 

 passed, at the beginning of our era, as a new star, then first registered. 

 And indeed the Arabian astronomers, one thousand years later, call 

 this star "The Forgotten," plainly because it had not been noticed 

 previously. 



We have a like instance in the star Alpha in Capricorn. Mankind 

 had to observe this star for thousands of years before they saw, what 

 any child may see when its attention is directed to the object, that 

 here are two stars (one of the third and one of the fourth magnitude) 

 so close together as to coalesce into one when hastily viewed. Again, 

 it was the Arabians who noted this circumstance. Still, this did not 

 avail to establish the true nature of a Capricorni. Ulugh Beigh, in 

 the fifteenth century, and Tycho Brahe, in the beginning of the seven- 

 teenth, in their famous " Catalogues of the Stars," take no notice of 

 it, and it was not till one hundred years later that Hevelius formally 

 entered the companion-star in his list. We cite two or three further 

 instances to show how the idealistic bias of the ancients, which cul- 

 minated in Aristotelism, has almost down to our own times diverted 

 men from simple but correct views of the world of sense. 



The amazing progress of observational astronomy during the last 

 two centuries is in great measure due to the hippy accident of our 

 hemisphere containing a bright polar star. Sundry investigations 

 can be made only with regard to stars near the pole, and all the more 

 easily, of course, and with smaller instruments, the larger the star 

 happens to be. The importance of this star impressed itself upon 

 men in former times, it being employed for correcting the compass. 

 And yet even Columbus was not clear whether Polaris is situated at 

 the north-pole, or only near to it, though it must be observed that in 

 his day its distance from the pole amouted to more than three degrees, 

 i. e., about six diameters of the full moon, and that hence it could 



