March, 1907.] 



THE TICTOEIAX NATURALIST. 



229 



list of some of the surrounding stars with their magni- 

 tudes, for example, the following stars in Carina, 

 namely — 



Upsilon, magnitude 3.4; omega =: 3.7 ; a ■=. 3.9; 

 I =: 4.3 ; n — 4.7, &c, 



Then finding, for example, that the star lies in bright- 

 ness between a and i, it is possible to estimate itsi 

 brightness to the tenth of a magnitude, at least after 

 some practice at the work. In this way, it was 

 easily found at the camp that, on the successive nights 

 of 25th, 26th, 27th, and 28th December, the magnitudes 

 of I Carinse were as nearly as possible 2i-^> 3-75 3-S> 

 3.9. The star varies from magnitude ^^-d to 5.2 and 

 back in about thirty-four days. Put in another way, it 

 means that the star was giving us just about four times 

 as much light on 25th December as it would be about 

 seventeen days after. Other stars go through changes 

 more or less complicated in much shorter times. For 

 example, not far from I Caringe a rather fainter star, 

 Y Puppis, is easily observable, by a practised observer, 

 to go through a complete series of changes in thirty-five 

 hours, and in that time, starting from the minimum 

 brightness, it increases to a maximum, falls off a little 

 again, increases to its maximum, and goes down to its 

 original minimum. Its brightness is represented gra- 

 phical Iv thus — 



Magnif-ude 



'f'/-- 



i-'i- 



4-7 



a 



h'me 



^^ ^^ ^^ in hours 



It is, consequently, easy to observe the change in bright- 

 ness in the course of two or three hours, if properly 

 observed with a field glass. In this case, the changes 

 are doubtless due to the presence of two stars very close 

 to one another, revolving and mutually eclipsing one 

 another. But this is not the explanation in the case of 

 I Carinae, of Avhich indeed, as well as of the great 

 majority of variable stars, the explanation is unknown. • 



