172 Proceedings of the Royal Irish Academy. 



16. E Ceti. — According to Schonfelcl, tlie period derived from 9 

 maxima is 167'65 days. 



17. T Arietis. — Period derived by Schonfeld from 2 Maxima and 

 2 Minima. Variation slow. 



18. jo Peesei. — Used as a comparison star for Algol, and in this 

 way found to be variable by Schmidt, 1854. It seems to have no 

 regular period, although Schmidt at first deduced a period of 33 days. 

 Schonfeld observed a very bright phase (equal to or greater than 

 S Persei) on the following dates : — 1854, October 30; 1866, Novem- 

 ber 28 ; 1867, February 2 ; 1869, March 9 ; and 1873, January 14. 



19. /3 Persei (Algol). — This well-known variable star is now 

 pure white, but seems to have changed in colour, as it was called red 

 by the Persian astronomer, Al-Sufi, in the middle of the 10th century. 

 Its variability was remarked by Montanari so early as 1667 or 1669, 

 and, after him, by Maraldi, Kirch, and Palitzsch ; but the true charac- 

 ter of its variations was first determined by Goodricke in 1782. Its 

 period, according to Schonfeld, is 2^ 20^ 48"" 53^-67 ; but this seems to 

 have diminished from 2'' 20^^ 48"" 59^-5, which it had in 1782. Por the 

 greater portion of its period it remains at a maximum of 2*2 m ; and 

 only during 9 J hours does its light fluctuate, with a minimum of 3'7 m. 

 about the middle of the period. The star has been especially studied 

 by Argelander, who found that the period is not quite constant, but 

 varies to a small extent. Schmidt finds that the brightness of Algol 

 is equal to that of 8 Persei about 47 minutes before and after mini- 

 mum ; to that of e Persei about 62 minutes before and after the same ; 

 and to that of y8 Trianguli, 95 minutes before and after. Prom his 

 observations between 1840 and 1875, Schmidt finds a period of 

 2d 20^^ 48"^ 53^-6. 



Schonfeld states that to his eye the variation of Algol is included 

 between the magnitudes 2-2 and 3*7. 



Prom photometric measurements of the light of Algol at maximum 

 and minimum, Professor Pickering finds that the diminution of light 

 commences about 263 minutes before minimum, and recovers its 

 normal brightness 337 minutes afterwards. The most rapid diminution 

 takes place about 100 minutes before, and the most rapid increase about 

 100 minutes after, the minimum. Pickering believes that the decrease 

 in the light of this star is due to a dark eclipsing satellite. 



I observed a minimum of Algol during totality of the lunar eclipse 

 of 23rd August, 1877. 



20. Pt Persei. — Prom numerous observations through 21 periods, 

 Schonfeld finds a period of 208'5 days. The Elements show an error 

 sometimes amounting to 16 days. At minimum the star remains for 

 about two months below 12 m., and sometimes descends even below 

 13 m. 



