Double Stars. 



147 



which, it would be most easily recognised. It is always deter- 

 mined by measuring the angle between two lines, one, the 

 parallel of declination passing through the larger star, or in 

 other words, its apparent course through the field of the tele- 

 scope ; the other, a line drawn from the larger to the smaller 

 star. Astronomers estimate the positions of all objects near 

 one another in this way, and amateurs, unprovided with the 

 convenient apparatus, called a position micrometer, will do well 

 to master thoroughly the accompanying diagram. 



It represents the field of an astronomical eye-piece, in which 

 objects being inverted, the N., where the measures commence, 

 will be found at the bottom, the S. at the top : the line joining 



them is a portion of a meridian ; the one at right angles to it 

 is a parallel of declination, along which (or parallel to which) 

 all objects move across the field from right to left, or from the 

 word ' ' Following " towards " Preceding." The initials n. p, 

 n. f, s. p, and s. f, which are constantly used in astronomical 

 works, stand for north preceding, north following, south pre- 

 ceding, and south following, and characterize the quadrants in 

 which they are respectively placed, so as to show at once, and 

 in a most convenient way, the general bearings of objects, 

 where accuracy of measurement is not required. In describing 

 a double star, the larger of the two, if differing in size, is sup- 

 posed to be placed in the centre of the field, and the position 

 of the lesser one, which, if very small, is often called a comes, 



