Work for the Telescope. 273 



Stanford, Charing Cross, can be strongly recommended, both 

 for accuracy and extraordinary cheapness. Their chief defect 

 arises inevitably from the attempt to represent a sphere by a 

 cube, so as to comprehend the whole in six maps ; in conse- 

 quence of this the corners become very much distorted, and 

 the squared-out degrees of the centre are there transformed 

 into a kind of lozenge-shape ; this must be carefully borne in 

 mind in comparing those parts with the heavens. A Moveable 

 Planisphere, by Smith, Strand, is also spoken of as very useful 

 in showing the principal stars above the horizon at any given 

 time. 



We may now proceed to our little working catalogue. 

 1. ZUrsceMajoris. Mizar. Distance 14"*4. Position 147 0, 4. 

 Magnitudes 3 and 5. Colours, white and pale green. Struve 

 makes them both greenish white. Relatively stationary, but 

 probably with common proper motion. There are several 

 reasons why this star may properly head the list. It is very 

 readily found, being the sixth in order of the seven stars of the 

 Great Bear, or the centre one of the tail; it is at a sufficient 

 height above the horizon during the whole year (though unfor- 

 tunately rather awkwardly elevated during the spring months) ; 

 it is within the grasp of very small instruments ; and from its 

 association with a third star, called Alcor, bright enough and 

 distant enough from the irradiation of its neighbours to be 

 visible without a telescope, it furnishes a peculiarly apt means 

 for training an untutored eye. Amateurs may be expected to 

 have at first confused and inadequate ideas of magnifying power; 

 and those who have never before viewed double stars with the 

 telescope will sometimes even imagine that they can distin- 

 guish them without it, though it is found that, for this purpose, 

 they ought, generally speaking, to be four minutes of space 

 asunder, a distance from eight to eighty times greater than 

 that of the generality of stars in the present list. Another 

 source of mistake may be that the great increase of brightness 

 in the telescope does not render itself at once intelligible to 

 the inexperienced eye. Erroneous impressions from these 

 causes may be effectually corrected by means of Mizar and 

 Alcor. Let these two be alternately viewed, in and out of the 

 telescope, a low power being used to include Alcor in the field, 

 and the inversion in the instrument being borne in mind ; and 

 it will be readily perceived how great is the effect of magnifying 

 power in separation, as well as of aperture in increased bright- 

 ness. It will be at once seen that no unaided eye could possibly 

 separate Mizar into its components, and the idea will never 

 be revived of the possibility of distinguishing double stars with- 

 out the telescope, excepting under circumstances of unusual 

 distance. 



