Page:Light waves and their uses.djvu/145

 LECTURE VII

APPLICATION OF INTERFERENCE METHODS TO ASTRONOMY

knowledge of the heavenly bodies is still very limited. The little that we have learned has been acquired almost entirely with the assistance of the telescope, or the telescope compounded with the spectroscope. Without these, the stars and the planets would always remain, even to the most perfect unaided vision, as simple points of light. With these aids we are every year adding very much to our knowledge of their constitution, their form, their structure, and their motions. For example, the spectroscope gives information concerning the elements contained in the sun and the stars; for by means of the dark or bright lines in the spectrum we are able to identify elements by the position of their spectral lines, and from this identification we are able to infer, with almost absolute certainty, the presence of the corresponding material in the heavenly body which is examined. The same is true of comets and nebulæ. By the general character of the spectrum we may also distinguish whether these bodies are in the form of incandescent gases, or whether they are in solid or liquid form; and we can, to a certain extent, infer their temperature. We can even determine whether the body is approaching or receding. For example, if the body is approaching, the waves are crowded together so that their wave length will be shortened, and hence they have a correspondingly altered position in the spectrum, i. e., the line will be shifted toward the blue end of the spectrum. If the body is receding, the spectral line is shifted in position toward the red end of the spectrum.