556 The Genesis of Double Stars 
The coincidence between the spectroscopic and the photometric 
evidence permits us to feel complete confidence in the theory of 
eclipses. When then we find a star with a light-curve of perfect 
regularity and with the characteristics of that of Algol, we are justified 
in extending the theory of eclipses to it, although it may be too 
faint to permit of adequate spectroscopic examination. This extension 
of the theory secures a considerable multiplication of the examples 
available for observation, and some 30 have already been discovered. 
Dr Alexander Roberts, of Lovedale in Cape Colony, truly remarks 
that the study of Algol variables “brings us to the very threshold of 
the question of stellar evolution.” It is on this account that I 
propose to explain in some detail the conclusion to which he and some 
other observers have been led. 
Although these variable stars are mere points of light, it has 
been proved by means of the spectroscope that the law of gravitation 
holds good in the remotest regions of stellar space, and further it 
seems now to have become possible even to examine the shapes of 
stars by indirect methods, and thus to begin the study of their 
evolution. The chain of reasoning which I shall explain must of 
necessity be open to criticism, yet the explanation of the facts by 
the theory is so perfect that it is not easy to resist the conviction that 
we are travelling along the path of truth. 
The brightness of a star is specified by what is called its “magni- 
tude.” The average brightness of all the stars which can just be seen 
with the naked eye defines the sixth magnitude. A star which only gives 
two-fifths as much light is said to be of the seventh magnitude; while 
one which gives 2} times as much light is of the fifth magnitude, and 
successive multiplications or divisions by 24 define the lower or higher 
magnitudes. Negative magnitudes have clearly to be contemplated ; 
thus Sirius is of magnitude — 1°4, and the sun is of magnitude — 26. 
The definition of magnitude is also extended to fractions; for 
example, the lights given by two candles which are placed at 100 ft. 
and 100 ft. 6 in. from the observer differ in brightness by one- 
hundredth of a magnitude. 
A great deal of thought has been devoted to the measurement of 
the brightness of stars, but I will only describe one of the methods used, 
that of the great astronomer Argelander. In the neighbourhood of the 
star under observation some half dozen standard stars are selected of 
known invariable magnitudes, some being brighter and some fainter 
than the star to be measured ; so that these stars afford a visible scale 
of brightness. Suppose we number them in order of increasing bright- 
ness from 1 to 6; then the observer estimates that on a given night 
his star falls between stars 2 and 3, on the next night, say between 
1 Proc. Roy. Soc. Edinburgh, xxtv. Pt. 11. (1902), p. 73. 
