148 Double Stars. 



or attendant, is given, if roughly, by the quadrant in which it 

 stands; if accurately, by the degrees included in the angle 

 already referred to, which is called its angle of position. 



In estimating angles of position, for the purpose of tracing 

 orbital motion, or knowing in what direction to look for very 

 minute attendants, the inexperienced observer may at first be 

 much puzzled, and frequently misled, by the varying situation 

 of the N. point, or zero of the diagram, according to the quarter 

 of the sky in. which the object is situated at the time. From 

 the amount of elevation of the pole in our latitudes, the transits 

 of stars through the field may be performed at angles of every 

 degree of inclination in one part or other of the sky, and the 

 line which stands horizontally in our diagram will be sensibly 

 horizontal in the heavens only when the object is in, or near, 

 the meridian of the place ; in the circumpolar regions of the 

 sky it may even become perpendicular to the horizon : in addi- 

 tion to which, the observer's head is frequently so twisted, 

 especially in using an achromatic telescope, that all accustomed 

 ideas of the position of objects are thrown into confusion. It 

 will be necessary, therefore, for the student to attend carefully 

 to the course of the principal star through the field, which 

 gives the parallel of declination ; to estimate the position of the 

 smaller object by referring to the diagram ; and to compare 

 this estimation with the measured angle : the eye will thus 

 require a training which will prove very serviceable. If he 

 possesses the habit of observing with both eyes open (a very 

 desirable one, as producing less uneasiness and fatigue), a glance 

 with the unarmed eye at the polar star, if within convenient 

 distance, will help to fix the line of the meridian ; from the 

 inversion in the eye-piece, the pole will of course stand at 

 180° in the diagram. 



4. Colour. — This, when fully developed, and especially when 

 strongly contrasted, adds a charm to many a pair otherwise 

 less interesting from the relative fixity of its components. 

 There is, however, a good deal of uncertainty about it, as there 

 is some diversity in the pictures in different instruments, and 

 much discrepancy in the estimates by different observers. The 

 achromatic, from its inherent defects, the uncorrected or " out- 

 standing" colours, which cannot be united with the rest, neces- 

 sarily gives an image whose tint is minus that outstanding 

 colour ; thus, as the latter is usually blue or purple, a white 

 star will be found to have a yellowish or slightly orange cast. 

 Reflectors are in theory exempt from this imperfection, but 

 unless the speculum is of perfectly white metal, and free from 

 tarnish, they are apt to give a smoky hue. From this cause, 

 or from some peculiarity of vision, Sir W. Herschel was so 

 partial to ruddy tints, that his observations, in this sole respect, 



