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67 



ETHER WAVES AND THE MESSAGES THEY BRING 



of 700 miles per second. Also it has been used to estimate the speed of stars 

 whose distances from the earth is entirely too great for it to be gaged by any 

 other method. A study of the stars from the point of view of this principle 

 has revealed the fact that on one side of the sun a majority of the stars seem to 

 be approaching it, while on the other side a majority are receding from it. This 

 can be interpreted only as evidence that the sun is really moving through space 

 toward the side on which the stars seem to be approaching it. Also this prin- 

 ciple has helped to reveal the structure of some of the heavenly bodies; for 

 instance, Maxwell, by means of mechanical considerations, showed in 1859 

 that the rings of Saturn could not be solid. The spectroscope reveals the fact 

 that they are either solid or liquid. It remained for Keeler to show, in 1895, 

 by the application of Doppler's principle, that the inner edge of the rings is 

 moving faster than the outer edge. As Maxwell had indicated, the rings are 

 composed of meteorites each moving separately as a satellite about the planet. 

 So much in a very hurried way for Doppler's principle and its appHcation 

 to ether waves. There is another device for reading the messages brought by 

 the visible waves which we ought to consider, then we will pass on to the in- 

 visible spectrum. The device to which I refer is the spectrohehograph. Let 

 us suppose that a telescopic image of the sun be formed in the plane of the sHt 

 of a spectroscope. Obviously the slit may be placed in any portion of the 

 image, and when in any particular portion, it will admit into the spectroscope 

 light only from that part of the sun whose image falls on the sHt. If there is, 

 for instance, calcium vapor in that portion of the sun, then the calcium Hues 

 produced in the spectroscope will reveal that fact. Now let us suppose that 

 the spectrum formed by the spectroscope be received on a screen and a second 

 slit be made directly in one of the calcium lines. Then a photographic plate 

 placed in this second slit will have formed on it an image of that part of the 

 image of the sun which falls on the first sHt, provided that this particular wave 

 length is being produced by the corresponding portion of the sun. Now if 

 sHt No. I be moved slowly across the image of the sun, and at the same speed 

 the plate behind sHt No. 2 be moved sidewise, there will be found on the plate 

 a continuous image of the portions of the sun which have given Kght to the 

 first slit. This image of the sun will represent it as photographed by the 

 particular wave length of the calcium Hne which has been used. In other 

 words, it plots out the areas of the sun which have calcium producing the 

 particular wave length used. In the same way, if sht No. 2 be made in a 



