Double Stars. 375 



tlie other upper corner is j3 Pegasi or Scheat ; the star beneath 

 ft is a, Markab ; <y Pegasi occupies the left corner below. It 

 may assist the student in acquiring the useful practice of esti- 

 mating degrees, if he knows that the length of the bottom of 

 the square is 17% and of the right side is 13% the top being 

 about 16% and the left side 14°. If from a Pegasi we draw a 

 hue back to our old acquaintance Delphinus, it will pass some 

 way above a solitary star, the brightest in a considerable region. 

 This is— 



65. e Pegasi. 2' 18"'l. 324°-3. 2|and9. Bright yellow 

 and blue lilac. A 14 mag. star at 1' 25" and 327° makes this 

 a triple group with a sufficient aperture. The object is not in 

 itself a remarkable one, but is inserted here as a striking ex- 

 ample of a phenomenon which seems to have been first noticed 

 by Sir J. Herschel. He found that when two unequal stars, 

 as of 4 and 9 mag., are situated at a moderate distance (10" to 

 60" or 80"), nearly in a vertical line, on giving the telescope a 

 swinging motion in a horizontal direction, the image of the 

 small star will oscillate, like a ball hung by a string, through 

 an arc greater in proportion to the difference of brightness ; 

 sometimes as much as 15° or 20° on each side of the vertical. 

 This he thinks is owing to the longer time which may be re- 

 quired for a feebler light to affect the retina, whence the change 

 of motion in a bright point may be more speedily perceived 

 than in a faint one. e Pegasi is an excellent instance of this 

 optical deception : it will, however, be too late to see it well at 

 the present season, as the position of the small star renders it 

 vertical before passing the meridian, which it does at about 5h. 

 in the beginning of December; but we shall remember it 

 another year. 



A diagonal of the square, from a Andromedce through a Pe- 

 gasi, bent a little downwards, points to a small "caltrop" or 

 triangle of stars, with a fourth in its interior. This inner star 

 is — 



66. f Aquarii. 3"-5. 352°-4 (1838-04). 3"-2. 346-9 

 (1852*81). 4 and 4^. Flushed white and creamy. There can 

 be no doubt of the binary character of this beautiful pair, whose 

 connection was discovered by Herschel I. in 1804. Its period 

 may possibly be about 750 years ; but an accurate determination 

 is yet wanting. Secchi gives 3"-328 and 344°-03 (1856-835). 

 It is within the reach of small apertures. I have seen it per- 

 fectly with 2f inches, and less would no doubt have sufficed. 



Aline from e Pegasi through the last object, carried as far 

 again, falls on a group of three small stars close together ; the 

 one to the right is — 



67 . ty 1 Aquarii. 49"-5. 310°- 5 . 5| and 9. Topaz yellow 

 and cerulean blue. In fine contrast, but apparently stationary. 



