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 neighborhood, 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 conditions, 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 seventeenth, 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 culminated 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
