24 Transactions of the South African Philosophical Society. 



produce the best results, viz., Polaris, a star in another part of the 

 sky, was photographed alongside /3 Persei for comparison. 



In Dr. Gill's photographs of S. Velorum the comparison stars are 

 those in the same field, and are consequently all photographed under 

 the same conditions of atmospheric absorption and instrumental 

 adjustments. 



Further, the same nine comparison stars that are imprinted on the 

 photographic plates are also adopted at Lovedale as standard stars 

 for the eye estimates of S. Velorum, and this mutual co-operation 

 and agreement as to the standard stars to which all changes in the 

 variable will be referred, cannot but have an important bearing on 

 the final determination of its variation. 



Especially will it have a direct bearing on the important question 

 of the velocity of light of different wave lengths ; that is if wave 

 lengths which mainly operate in producing the sensation of vision are 

 different from those that leave their impress on the photographic plate. 



S. Velorum also is the first variable of its class discovered by 

 photography, and the second Algol variable — as the class to which 

 S. Velorum belongs is called — -discovered in the Southern Hemi- 

 sphere. Indeed the ceaseless activity in this branch of astronomy 

 has only resulted in the discovery of fourteen Algol variables, of 

 which four are in the Southern Hemisphere. 



This star, therefore, has no ordinary claims upon our attention, 

 and the purport of the present paper is to state as far as has been 

 ascertained from eye estimates alone the chief features of its variation 

 and their probable explanation. 



II. — Light Curve of S. Velorum. 



During 5 days 7 hours S. Velorum remains constantly a 7'75 

 magnitude star. An ordinary opera-glass will show it distinctly 

 during this period of constancy. At the end of this period it begins 

 to decline in brightness, slowly at first, and then with ever-increasing 

 rapidity. As it passes the 9th magnitude, the limit of vision in a 

 one-inch telescope, its rate of descent is one-tenth of a magnitude in 

 eight minutes. When the variable, however, reaches magnitude 

 9-25 it comes to a sudden halt. For the next six and a half hours it 

 remains constant at this magnitude. 



At the end of this time it suddenly and rapidly begins to ascend 

 again. As it increases in brightness the rate of increase slows down, 

 till, when it nears its normal magnitude, its rate is so slow as to 

 render it extremely difficult to say when the increasing phase ceases. 



On reaching magnitude 7*75 the constant period sets in, and 



