Page:Popular Science Monthly Volume 2.djvu/678

658 spectrum—uninterrupted by lines—is produced, when the light emanating from solid and liquid incandescent bodies is passed through a prism. 2. A spectrum interrupted by bright lines is produced when the light emanates from flames or ignited vapors and gases. 3. A spectrum interrupted by dark lines is produced, when light emanating from a source giving a continuous spectrum, is passed through gaseous or vaporous matter giving spectra of the second order.

Now, it has been found that, when various elements are volatilized in the flame of a lamp, the light gives a spectrum interrupted by bright lines—whose character and position are different for different elements. It has also been discovered that the dark lines of spectra of the third order correspond precisely in position with the bright lines in spectra of the second order: they thus indicate the existence of elements which are volatilized in the ignited vapors or gases. The coincidence of position of these bright and dark lines was first observed and described by Foucault, of Paris, in 1849; but their real significance was first indicated in 1859, by Kirchhoff, of Heidelberg. These delicate lines carry across the immeasurable abysses of the celestial spaces evidences of their origin!

The numerous lines of the spectrum are separated from one another—the fan of light is opened out—its entire pattern is brought distinctly under view—and all of its minute details are revealed—by transmitting the light through a succession of prisms: this constitutes the Spectroscope. (This was illustrated by a diagram.)

By means of the spectroscope, no less than fourteen terrestrial elements have been identified as existing in the sun's atmosphere. Mr. William Huggins and Prof. W. A. Miller, by ingenious modifications of this instrument, have been able to extend spectrum analysis to more than sixty of the brighter fixed stars. Like our sun, they give spectra with dark lines; thus indicating that the stars (as the sun) must have intensely heated solid or liquid nuclei, surrounded by ignited gaseous atmospheres.

Encouraged by his success with the fixed stars, Mr. William Huggins applied the potent method of spectrum analysis to the examination of the nebula?. He was rewarded by a most important discovery in relation to the physical constitution of these wonderful objects. On the 29th of August, 1864, he applied his spectroscope to a planetary nebula in Draco. He was astonished to find that there was no appearance of a band of colored light, such as a star would give; but, in place of this, there were three isolated bright lines on a dark ground—a true gaseous or vaporous spectrum. In other words, the object was not a cluster of stars, but a true nebula. Mr. Huggins was not slow in following up this line of investigation. During the two years succeeding his first observation, he examined the spectra of more than sixty nebulæ and clusters. Of this number, about twenty gave spectra with bright lines; that is, were gaseous bodies. The remaining forty