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been said the reader will see that, using words in their ordinary sense, we do not see anything by the aid of a spectroscope. What we do with it is to analyse the rays of light into their component parts, just as a chemist analyses a compound body into its simple elements. A spectroscopic analysis is more complicated from the fact that the number of elements which compose a ray of light is generally indefinite. The great advantage of spectroscopic analysis arises from the fact that it is independent of distance. The farther a star is away, the more difficult it is to see, whether we look at it with the naked eye or through a telescope. Its light diminishes as the square of the distance increases; twice as far away it gives us only one fourth the light; three times as far away, only one ninth the light, and so on. But if enough light comes from the star to enable its spectrum to be analysed, the result can be reached equally well no matter how great the distance. As the chemist could analyse a mineral brought from the planet Mars, were such a thing possible, as easily as he could if he found it on the earth, so, when a ray of light reaches the spectroscope, the fact that it may have been hundreds of years on its way, does not interfere with the drawing of conclusions from it.

When the spectrum of a star is formed it is always found to be crossed by numerous dark lines. This shows that all the stars, like our sun, are surrounded by atmospheres which are not as hot as the central body. But this does not imply that the atmosphere is cold. On the contrary, it is probably hotter than the flame of any furnace we have on earth, even in the case of the cooler stars.