PRESIDKN TtAr, ADDRKSS SI-XTION B. ^f 



and since the spectra of the known elements are known, it is 

 pctssible to identify the lines of characteristic elements present in 

 bodies emitting H^ht of their own, or present in vapours through 

 which light passes. The delicacy of spectroscopic analysis is 

 very great, and it is possible, for instance, to detect one part of 

 sodium vapour in twenty thousand million parts of air. 



By turning the telescopic spectroscope on to the stars it has 

 been shown that, although most of the well-known elements are 

 there, some elements are present (unknown bands) which have not 

 yet been discovered on the Earth, and have never been handled in 

 our laboratories. The element Helium, which has recently 

 become of great theoretical importance, and will be of great 

 econonu'c importance if it can be obtained in sufficient quantity, 

 was first detected in the sun and actually named before it was 

 discovered on the Earth. By studying the absorption bands in 

 the solar spectrum the atmosphere of the Sun has been studied, 

 and the presence of about 40 terrestial elements discovered. 



By applying the telescopic spectroscope in another way it 

 is possible to determine a "velocity" instead of a substance, 

 and to determine the speed of stars moving in the line of sight. 

 This is done by accurately comparing the position of certain 

 lines produced by elements in the star with the position of the 

 lines produced by the same elements in the neighbourhood of 

 the spectroscope. Thus, for instance, if the hydrogen or iron 

 line of a star is nearer the violet end of the spectrum than it 

 should be, the star is coming towards us ; if nearer the red end, 

 the star is travelling away from us. By photographing stellar 

 and terrestrial spectra of the same element on the same plate, 

 and measuring the deviation with a microscope, it is possible to 

 calculate the approximate speed at which Star and Earth are 

 approaching to, or receding from, each other. 



What is really being done is to measure the frequency of the 

 waves of light from the star, since the position of any line in 

 the spectrum is conditioned by this. A rough analogy which 

 has been used to explain the process to the lay mind, is offered 

 by the consideration of a person standing on the seashore with 

 the waves breaking over his feet at a steady rate — so many per 

 minute. If he now^ walks into the sea, he will encounter more 

 waves per minute, and the faster he moves the more frequently 

 will the waves appear to break over him. If he then walks 

 back to the shore, the waves will appear to come more slowly 

 and fewer will break over him in any given time. In reality 

 the waves are coming at the same rate all the time, but by 

 m.easuring the frequency with which they appear to come to him 

 he could calculate the rate at which he was walking. In the 

 same way, by measuring the deviation of frequency of the light 

 v/aves coming from a particular element in a star, from the 

 known frequency on the Earth, or, in other words, measuring 

 the displacement of its spectral lines, the speed of the star in 

 the line of sight can be calculated. 



