OF ARTS AND SCIENCES. * 259 



From this table it appears that only six stars of the fifth class are 

 as yet known. Although the published observations of some of the 

 others are insuiricioiit to determine the nature of their variations, it is 

 probable that most of them belong to the fourth class. The first star 

 on the list, which is DM. 81°.25, has been designated as T Cephei,* but 

 the use of this name has created much confusion. In 18G3,t Arge- 

 lander announced the variability of the star DM. 55°. 2943, and this 

 star is called T Cephei in Chambers' Astronomy, p. 586. In 1871), 

 Ceraski t announced that DM. 67°. 1291 was variable. When cor- 

 rectuig its position § he called it T Cephei. This correction is quoted 

 in the " Astronomical Register," xviii. 322, under the heading " W. 

 Ceraski's new Variable," apparently confounding it with Ceraski's 

 last discovery, DM. 81°.25. 



The most natural explanation of the variation of a star of short 

 period is that it is due to its rotation around lis axis. 



In the Annals of the Harvard College Observatory, xi. 264, the 

 variation in light of lapetus, the outer satellite of Saturn, is discussed 

 on this hypothesis. It is there shown that if tlie axis of revolution is 

 perpendicular to the line of sight, the variation of light, L, may be 

 approximately represented by the formula, L =^ a -\- h ?kw v -\- c 

 eos V -|- c? sin 2 w -]- e cos 2 v ; « here denotes the mean light, v the 

 angle of rotation, h and c are constants depending on the comparative 

 brilliancy of the two hemispheres, each of which is supposed to be of 

 uniform intensity, but one brighter than the other ; d and e depend on 

 a supposed deviation of the body from the form of a solid of revolu- 

 tion. This equation may also be written in the form Z = a -f- m 

 sin (y -|- «) -|- « sin (2 v -(- /3), in which a depends upon the angular 

 position of the plane separating the two hemispheres from the line of 

 sight at the epoch from which the variation in light is reckoned ; ^ in 

 like manner depends upon the positions in which the body subtends 

 its largest and smallest discs. Our problem then is to see how far 

 this equation will represent the variation in light of all the stars of 

 the fourth class. 



Both of these proposed causes of variation may be criticised as 

 improbable ; but what could be more improbable than the phenomenon 

 itself, were it not verified by observation ? With our present knowl- 



* Science Observer, iii. .SO, 38, 48. English Meclianic and World of Science, 

 xxxii. 2'j7. Astron. Nach. xcix. 87. 

 t Astron. Nach., Ixi 281. 

 X Astron. Nach., xciv. 175. 

 § Astron. Nach , xcviii. 239. 



