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CELESTIAL SPECTROSCOPY. 95 
two-hundredth part of a second of arc, and therefore very far too small 
for the highest powers of the largest telescopes. If we take the rela- 
tion of aperture to separating power usually accepted, an object glass 
of about 80 feet in diameter would be needed to resolve this binary star. 
The spectroscope, which takes no note of distance, magnifies, so to 
speak, this minute angular separation 4,000 times; in other words, the 
doubling of the lines, which is the phenomenon that we have to observe, 
amounts to the easily measurable quantity of 20 seconds of are. 
There were known, indeed, variable stars of short period, which it 
had been suggested might be explained on the hypothesis of a dark 
body revolving about a bright sun in a few days, but this theory was 
met by the objection that no such systems of closely revolving suns 
were known to exist. 
The Harvard photographs of which we have been speaking were 
taken with a slitless form of spectroscope, the prisms being placed, as 
originally by Fraunhofer, before the object glass of the telescope. This 
method, though it possesses some advantages, has the serious draw- 
back of not permitting a direct comparison of the star’s spectrum with 
terrestrial spectra. It is obviously unsuited to a variable star like 
Algol, where one star only is bright, for in such a case there would be 
no doubling of the lines, but-only a small shift to and fro of the lines 
of the bright star as it moved in its orbit alternately toward and from 
our system, which would need for its detection the fiducial positions of 
terrestrial lines compared directly with them. 
For such observations the Potsdam spectograph was well adapted. 
Prof. Vogel found that the bright star of Algol did pulsate backwards 
and forwards in the visual direction in a period corresponding to the 
known variation of its light. The explanation which had been sug- 
gested for the star’s variability, that it was partially eclipsed at regu- 
lar intervals of 68.8 hours by a dark companion large enough to cut off 
nearly five-sixths of its light, was therefore the true one. The dark 
companion, no longer able to hide itself by its obscureness, was brought 
out into the light of direct observations by means of its gravitational 
effects. 
Seventeen hours before minimum, Algol is receding at the rate of 
about 244 miles a second, while seventeen hours after minimum it is 
found to be approaching with a speed of about 284 miles. From these 
data, together with those of the variation of its light, Vogel found, on 
the assumption that both stars have the same density, that the com- 
panion, nearly as large as the sun, but with about one-fourth his mass, 
revolves with a velocity of about 55 miles a second. The bright star, 
of about twice the size and mass, moves about the common center of 
gravity with the speed of about 26 miles a second. The system of the 
two stars, which are about 5,250,000 of miles apart, considered as a 
whole, is approaching us with a velocity of 2.4 miles a second. The 
great difference in luminosity of the two stars, not less than fifty times, 
