Work for the Telescope. 275 



twice by liiinself and his friends. The discrepancies are 

 occasionally very strange, and some of them will be given in our 

 present list ; but, on the whole, many of the Italian observations 

 of that date seem more valuable as illustrations of indi- 

 vidual peculiarity than as evidences of fact. 



It has been suspected that Alcor, together with Mizar and 

 its companion, may be mutually connected as one grand ternary 

 system. Several minute stars will be found in the field, the 

 largest of the eighth magnitude. Mizar has of late years been 

 photographed no less than 86 times with the great achromatic 

 of 14f inches aperture at Harvard College, U. S., in order to 

 measure the image instead of the object — a much more conve- 

 nient process, which seems to promise some great advantages. 



2. a Ursce Minoris. Polaris. 18"' 6. 210° - 1. 2\ and 9|. 

 Yellow and dull white. So Struve. Sestini and Dawes make 

 9^ blue, and so I have thought it. This is the Pole-star, so 

 called, not from its occupying the polar point, from which it is 

 distant about 1° 25', or somewhat more than 2,\ diameters of 

 the sun, but from its being the nearest to it perceptible by the 

 naked eye. The mode of finding it by a long line passing 

 through the Pointers, or right hand stars of the Great Bear, is 

 well known, and once found it will not be lost, as it stands 

 comparatively alone in a wide space. The comes has long been 

 referred to as a test of the goodness of small glasses. It was 

 formerly proposed as a standard of eyes and instruments by 

 Dawes, who considered that in most instances two inches' of 

 aperture would be scarcely sufficient to show it steadily. 

 Smyth, however, saw it distinctly with his 5 T ^- -inch object- 

 glass reduced to that size. The celebrated Dorpat achromatic 

 of 9^ inches exhibits it even while the sun is above the horizon, 

 and Kitchiner saw it with a power of 18 applied to a 7-inch 

 object-glass by the elder Tulley. In the case of minute points 

 which approach the minimum visibile in a telescope, the device 

 of oblique or averted vision, by which the image is made to fall 

 on a more sensitive part of the retina, will often give the first 

 intimation of their existence ; and when their position is once 

 known, they frequently become much more perceptible. 



Polaris seems to be merely an optical pair, their proximity 

 being the effect of perspective alone, and their real distance 

 being incalculable and incomprehensible. We shall find, how- 

 ever, as we proceed, that magnitude is a much less safe criterion 

 of distance than was supposed in former days. 



A small amount of parallax has been ascribed to the large 

 star. If correct, it shows a distance which light, reaching us in 

 8j minutes from the sun, would not traverse in less than thirty 

 years. 



3. a Ursce Majoris. Dubhe. 6' 2(P6. 203°-8. l\ and 8. 



