276 Work for the Telescope. 



Both yellow. I have thought 8 violet with a 3 T ^- object-glass, 

 lilac with 5^ inches. This is the most northerly of the Pointers. 

 It is a very wide object, and of little scientific interest ; but it 

 appeared to me of sufficient beauty to merit insertion, especially 

 as the yellow hue of the large star is very fine. 



4. a Geminorum. Castor. 4"*7. 258°-8. (1830-95). 4"-9. 

 248-1. (1849-17). Fletcher found 5"-309. 245°-66. (1857-28). 

 3 and 3|. Bright and pale white; greenish, according to 

 Struve ; greenish yellow and greener, Dembowski. This is the 

 most northerly of the two large stars which mark the heads of 

 the Twins, and which will be found declining towards the W. 

 at the beginning of May. Castor is an intrinsically glorious 

 pair, and especially interesting as being, in Sir John Herschel's 

 words, "the largest and finest of all the double stars in our 

 hemisphere, and that whose unequivocal angular motion first 

 impressed on my father's mind a full conviction of the reality of 

 his long-cherished views on the subject of the binary stars." 

 That it is binary — a superb system of two neighbouring suns, 

 there can be no doubt ; but there is some difficulty in ascertain- 

 ing its period, although it has been watched from an earlier 

 epoch than most objects of this kind. Sir J. Herschel has 

 assigned 253 years, Smyth 240 ; the result is not entirely satis- 

 factory, but of the revolution round a common centre of gravity 

 there is no doubt. "This," as Admiral Smyth says, "is a 

 great fact, and an astronomical revelation which, in all probability, 

 Newton himself never contemplated." In the case of undoubted 

 binary pairs, more than one of Smyth's epochs will be given as 

 above, for comparison. The earliest observation, that of Bradley 

 and Pound in 1 719, made the angle 355-53 ; a veiy little experi- 

 ence in estimating positions will show how great has been the in- 

 termediate change. A power of 80 should bring out this pah' 

 well. I have divided it with 55, and an aperture of 5^ inches. 



5. ZOamcri. 5"-4. 149-4. (1832-23). 4-8. 144 J -1. 

 (1853*17). 6 and 7|. Both yellow. Sestini makes the small 

 star white ; yet he calls Castor yellowish and yellow. This easy 

 double star is apparently in slow motion, in an orbit of possibly 

 500 or COO years; none but very fine telescopes are now capable 

 of showing, what was more easily distinguishable twenty years 

 ago, that the larger star has another of seventh mag. almost in 

 contact with it. A very fine five feet achromatic may possibly 

 detect some elongation. This close pair is in rapid motion, but 

 with a period as yet undecided ; and the whole triple combina- 

 tion, if such it be, with all its intricate relations and movements, 

 is a truly wonderful object. To find it, run a line from Castor 

 through Pollux, and continue it between two and three times 

 the distance of those stars ; it will pass a little above £, when 

 W. of the meridian. 



