FREE INSTITUTE OF SCIENCE g 



ETHER WAVES AND THE MESSAGES THEY BRING 



called, is by means of the spectroscope. This is an instrument in which the 

 dispersed beam is introduced into a telescope, thereby enabhng the observer 

 to view the spectrum greatly magnified. By means of the spectroscope it has 

 been found that spectra are not all alike, but fall into three general classes, 

 each determined by the source from which the Hght has come and the treat- 

 ment it has subsequently undergone. For instance, if the light coming directly 

 from a glowing solid or hquid, such as the white-hot carbons of an arc light or 

 a mass of molten metal, be viewed through the spectroscope, the spectrum is 

 continuous from the red to the violet (Fig. i, ^). This simply means that 

 the light from such a source contains all possible wave lengths from 3800 to 



o 



7800 Angstrom units. This is called a continuous spectrum. If some gaseous 

 substance, however, is heated to incandescence and the light given off be dis- 

 persed, the spectrum is not at all continuous, but consists usually of a few bright 

 lines of color. This can only mean that but a few wave lengths are present in 

 such hght. This forms a spectrum such as you see on the screen, and is called 

 a bright Hne spectrum (Fig. i, C). Now a remarkable fact about this phase 

 of the subject is that each gas has its own set of lines which are characteristic 

 of that particular gas. Thus sodium always gives off yellow hght, which is 

 made up of two slightly different wave lengths, and consequently always pro- 

 duces a spectrum of two lines lying close together, known as the D hnes. In a 

 spectrum where the dispersion is moderate these hnes are so close together 

 as to appear as one hne (Fig. i, C). Hydrogen has a very complex spectrum, 

 but one that is entirely characteristic. In the same way calcium, potassium, 

 barium, iron, and, in fact, every other substance when volatilized and made in- 

 candescent, produces a characteristic bright hne spectrum. This is the fact 

 upon which the spectrum analysis of the chemist depends. He places in a hot 

 flame the substance to be studied, and it is heated to incandescence after having 

 been vaporized. Then passing the light through the slit of the spectroscope 

 he determines the material present by identifying the groups of hnes in the 

 spectrum thus produced with the lines known to be produced by the various 

 known substances. As may be imagined, this method of study has been a 

 powerful aid in determining the substances to be found in the sun and other 

 celestial bodies; for if any gas from a terrestrial source produces always a cer- 

 tain set of bright hnes and those hnes cannot be produced in any other way, we 

 are justified in assuming that when those same bright lines are produced by the 

 light from a celestial source, the same gas is present in that source. 



