320 DISCOVERY CH. 



but they can be detected by other effects. To an astro- 

 nomer with a spectroscope, the light of a star is a gamut 

 of colour crossed by dark or light rays comparable with 

 musical notes. Just as the pitch of a note can be 

 raised or lowered by rapid motion of the sounding body 

 towards or away from the listener, so the positions of rays 

 in the light-scale are affected by similar movements of 

 approach or recession. If the distance between the 

 moving star and the earth is decreasing, the rays analysed 

 by the spectroscope are increased in colour pitch, and if 

 the distance is increasing the rays are moved towards the 

 lower end of the gamut of light, or, expressed in the 

 terms of music, their notes are flat. So perfect a means 

 does the spectroscope provide of measuring the move- 

 ment, back or forth, that the velocity of a star can be 

 determined within a quarter of a mile a second, though 

 the star itself may be at an immeasurable distance from 

 us. Here, then, we are provided with another means 

 of studying stellar motions, and to it we owe the proof 

 of the existence of many dark stars. 



That such dark stars existed was first suggested by 

 John Goodricke, who, though deaf and dumb from birth, 

 used his sight to such good effect that his name is 

 renowned among astronomers. He died at the early age 

 of twenty -two, yet his observations, made in a small 

 observatory at York, obtained for him in 1783 the 

 Copley Medal. of the Royal Society, the highest honour 

 which the Society can confer. Goodricke was the first 

 to make a systematic study of the variations in bright- 

 ness of the star Algol a name derived from the Persian 

 word signifying the " demon." This star shines steadily 

 with a brightness equal to that of the Pole Star for nearly 

 two and a half days, and then suddenly its light is 

 reduced. In about four and a half hours the star's 



