408 



KNOWLEDGE. 



November, 1913. 



complex unity ; and the latest revelation of all that 

 the stars are telling us in the cipher messages is 

 that the scheme of creation is a cyclic system 

 infinite and immortal. 



the Physics of the Stars. 



The principle of isochromatic reversal was not 

 merely to tell the chemistry of the heavens, but, 

 joined to another fact called Doppler's principle, was 

 to unfold all kinds of physical facts and astronomic 

 wonders. 



doppler's principle. 



The express that took me to the British Associa- 

 tion Meeting at Birmingham passed another express 

 that had its steam whistle in full blast. As the 

 train approached the note was ghastly in the high 

 pitch of its shriek ; suddenly, as the engine passed, 

 the note dropped enormously, to be followed 

 instantly by a low-pitched groan. I had often 

 noticed the pitch of an insect's hum drop as it 

 passed me, and, standing on a railway bridge, had 

 heard the note of a steam whistle change as it 

 passed under the bridge. But the passing of those 

 two swift expresses gave me an illustration of 

 Doppler's principle that I have never heard equalled 

 before. 



Each vibration produced by the approaching 

 whistle was made in a position nearer to us than 

 the former impulse, so they were closed up and 

 became shorter ; a shorter wave produces a higher 

 pitch, and so the whistle shrieked ; when the train 

 had passed, and was going away, each vibration 

 originated at a more distant point and the waves 

 were lengthened, the note consequently dropped, 

 and the high shriek became a low groan. 



The same principle applies to colour: the slow 

 waves of light are red, the swift ones violet, the 

 other tints have intermediate velocities. Suns, when 

 very close to one another, have been pulled by mutual 

 gravitation, so that they may have speeds of 

 hundreds of miles a second ; the pitch of the definite 

 waves produced by each kind of their singing 

 atoms have the pitch of their definite waves of 

 light altered, and they become shorter as the 

 suns approach, and longer as they recede. 



Hence a pair of suns that in our most powerful 

 telescopes appear to be a single point of light may, 

 under the spectroscope, give us a spectrum in which 

 a characteristic line of hydrogen may appear double, 

 and each line be displaced from the normal position. 

 In such a double star the sun approaching us will 

 give us lines displaced towards the violet, and the 

 sun receding a line displaced towards the red end of 

 the spectrum. Thus the spectroscope reveals the 

 motion as well as the composition of heavenly bodies. 



If the pair of suns graze, a third body is struck off, 

 like the spark from flint and steel. In the case of 

 the exploding third star, produced in this way by 

 grazing suns, the quickly expanding and ensphering 

 shell of hydrogen that would be produced, as 

 described in "Knowledge" in September 1911, 



would be made up of atoms moving in all directions 

 away from the nucleus of the exploding sun, and 

 hence in all directions to our line of sight (see Figure 

 420). The atoms moving towards us would give lines 

 displaced towards the violet, and those away from us 

 towards the red. Intermediate directions would have 

 resolved velocities that would fill the space between 

 these. The single line of light that would be pro- 

 duced were the hydrogen at rest, would broaden 

 into blaze bands, occasionally showing an opposite 

 speed on the two edges of each band of thousands 

 of miles a second. In the special case of Nova 

 Persei, the displacement in each direction was fully 

 a thousand miles a second. This is perhaps the 

 most interesting historic example of the physical 

 reading of stellar spectroscopy. The details of these 

 two wonderful principles of reversal of light and 

 change of speed will be given in detail in the 

 succeeding articles. Figure 423 shows the dark 

 bands of hydrogen as seen in ordinary stars. 



THE ASTRONOMY OF THE INVISIBLE. 



This principle of relative motion of lines reveals 

 the existence of dead suns by their influence on 

 luminous stars ; the spectra of some stars are seen 

 to have their bands moving periodically backwards 

 and forwards on the spectrum. Such a pair of stars 

 are one class of what are known as spectroscopic 

 binaries, so called because it is only in the spectro- 

 scope that the fact that they are double stars is 

 revealed. Because in addition, as in the case of 

 Algol the Demon Star, the dark star sometimes 

 eclipses the bright one, we are able to deduce their 

 orbits, and thus weigh and measure these two 

 properties of the two constituents. 



SOLAR ECLIPSES. 



When the Moon exactly covers the face of the 

 Sun, as it did for a second or so last year, if we arm 

 the telescope with a prism, a series of images of the 

 circular rim of light is produced. There is a 

 different ring for each special note of the vibratory 

 atoms of the various metals. We thus get a most 

 singular spectrum covered with rings of light, 

 instead of straight lines; some of the rings overlap 

 one another and some stand clear. Probably it is 

 not more than once in a century that the disc of the 

 Moon and the Sun thus exactly overlap and leave the 

 reversing layer to be seen all round. There is only 

 one small spot on all the Earth (and that not 

 precisely known) where it can be seen. Its duration 

 is something like a second. Yet the photograph of 

 this rare and elusive phenomenon was actually taken 

 last year in Portugal by James H. Worthington. It 

 will be published in the next number of 

 " Knowledge." The same principle is used during 

 ordinary eclipses to photograph the great crimson 

 hydrogen flames and the metallic outbursts that 

 spring from the Sun (see Figure 419). 



By means of the same method of placing a large 

 prism in the path of the rays gathered by the 

 telescope it is possible to so dilute the ordinary 



