452 On the Origin of the Light of the Sun and Stars. 



tion of temperature. It is of course possible to imagine that 

 a peculiar cooling process takes place, so that the body of the 

 sun, originally very bright, is greatly reduced in temperature 

 when we behold it, but such an hypothesis bears the appear- 

 ance of patchwork, and even if it account for solar phenomena, it 

 will not admit of extension to other systems. From all this, we 

 are induced to suppose that the sun's light is due to action from 

 without ; and if it can be proved, as we think it can, that a disc 

 full of spots is deficient in luminosity, it would seem to follow 

 that such a state of the sun's surface implies a deficiency in the 

 intensity of this mysterious action ; while, on the other hand, a 

 disc free from spots denotes an increase of the same. 



If we now direct our adventurous flight into still more 

 distant regions, we shall find evidence of very extraordinary 

 forces at work in stellar spaces. We allude to variable, tem- 

 porary, and binary stars. Of the first and second of these classes 

 we shall here name one or two of the most prominent examples. 



1 . Omicron Ceti has its greatest brightness for a fortnight, 

 decreases for three months, is invisible five months, in- 

 creases again for three months, arriving once more at its 

 greatest brightness. 



2. Algol in Perseus appears for about sixty-two hours as 

 a star of the second magnitude ; it then suddenly becomes 

 fainter, and in three hours and a half arrives at its minimum ; 

 it then begins to revive, and in three hours and a half more is 

 again at its maximum brightness. 



3. Gamma Cygni is visible for about six months, and invi- 

 sible for about the same time or a little longer. 



The appearance of a temporary star about 125 years B.C., 

 which shone forth for some time with extraordinary brilliancy and 

 then died away, turned the attention of Hipparchus to astro- 

 nomy, and induced him to form a catalogue of stars. In tho 

 year 380 a.d., a star shono forth with extreme brilliancy near 

 Alpha Aipiilu', remained for three weeks as bright as Venus, 

 and then disappeared, A star of this kind was first seen by 

 Tycho Brahe in November, 1572. It was at its greatest bril- 

 liancy when discovered, diminished gradually in brightness for 

 iiiteen months, and disappeared in March, 157 1. There was 

 no change in its apparent place. Kepler also Baw a now star 

 i.ii the 8th of October, 1604, It had suddenly become visible, 

 was of great Lustre, and disappeared after twelve months. 



This is perhaps the fittest place to notice the behaviour of 

 binary stars, A binary star denotes a, system generally of 

 two members, which revolve about ono another in ellipses 

 frequently of great ('(cent ricity. A change of magnitude in tho 

 components of somo of theso systems has been observed, and, 

 as far as can bo gleaned from an interesting paper by Professor 



