$302] Solar Spectroscopy : Dpppler>s Principle 391 



Professor Harkness and Professor C. A. Young, who dis- 

 covered a bright line of unknown origin * in its spectrum, 

 thus shewing that it consists in part of glowing gas. 

 Subsequent spectroscopic work shews that its light is partly 

 reflected sunlight. 



The corona has been carefully studied at every solar 

 eclipse during the last 30 years, both with the spectroscope 

 and with the telescope, supplemented by photography, and 

 a number of ingenious theories of its constitution have been 

 propounded ; but our present knowledge of its nature hardly 

 goes beyond Professor Young's description of it as " an 

 inconceivably attenuated cloud of gas, fog, and dust, sur- 

 rounding the sun, formed and shaped by solar forces." 



302. The spectroscope also gives information as to certain 

 motions taking place on the sun. It was pointed out in 1842 

 by Christian Doppler (1803-1853), though in an imperfect 

 and partly erroneous way, that if a luminous body is 

 approaching the observer, or vice versa, the waves of light 

 are as it were crowded together and reach the eye at shorter 

 intervals tha?:i if the body were at rest, and that the character 

 of the light is thereby changed. The colour and the position 

 in the spectrum both depend on the interval between one 

 wave and the next, so that if a body giving out light of a 

 particular wave-length, e.g. the blue light corresponding to 

 the F line of hydrogen, is approaching the observer rapidly, 

 the line in the spectrum appears slightly on one side of its 

 usual position, being displaced towards the violet end of 

 the spectrum ; whereas if the body is receding the line 

 is, in the same way, displaced in the opposite direction. 

 This result is usually known as Doppler's principle. The 

 effect produced can easily be expressed numerically. If, 

 for example, the body is approaching with a speed equal 

 to y^oo^ that f n g nt 5 then 1001 waves enter the eye or the 

 spectroscope in the same time in which there would other- 

 wise only be 1000; and there is in consequence a virtual 

 shortening of the wave-length in the ratio of 1001 to 

 1000. So that if it is found that a line in the spectrum 

 of a body is displaced from its ordinary position in such 



* The discovery of a terrestrial substance with this line in its 

 spectrum has been announced while this book has been passing 

 through the press. 



