(A) A Bunsen flame is placed in front of the slit and a heated platinum wire which has been dipped in common salt or some sodium compound placed in the Bunsen flame; the latter becomes yellow and a vivid yellow line is observed on the screen in the spectroscope. Other substances, as barium and strontium salts, when heated to incandescence in the Bunsen flame, give characteristic bright lines. In fact each _element_ has been found to have its own characteristic set of colored lines. This fact is made use of in _spectrum analysis_, by which the presence of certain elements in a substance can be definitely proved upon the appearance of its particular lines in the spectrum. [Illustration: FIG. 407.--Diagram of a spectroscope.] [Illustration: FIG. 408.--The bright line spectrum of iron and its coincidences with some of the dark lines of the solar spectrum.] (B) If light from, for example, an arc light is sent over a gas flame containing _sodium_ vapor, a _dark line_ appears in the spectrum--in the exact position in which the yellow sodium line appeared. It seems that the sodium vapor removes from white light the same wave lengths that it itself produces. This absorption is supposed to be due to sympathetic vibration; just as a tuning fork is set in vibration by the waves of another fork in unison with it, at the same time absorbing the wave energy, so in the gas flame the sodium particles absorb the wave motion of the same vibration rate as that emitted by them. The fact that the spectrum of sunlight contains a great many dark lines is believed to indicate that the sun is surrounded by clouds formed by the vaporization of the various substances in the sun itself. By comparing the dark lines of the solar spectrum with the _bright-line spectra_ of various substances found in the earth, such an exact correspondence of the lines is found that the presence of the vapor of these substances about the sun is considered proved. (See Fig. 408 which shows the exact correspondence between the bright-line spectrum of iron vapor and the dark lines appearing in a portion of the sun's spectrum.) The spectra of the stars also contain certain dark lines. Thus the presence of the corresponding substances in distant stars is considered as determined. =408. Theory of Color Vision.=--By combining light of the _three colors_ _red_, _green_ and _blue-violet_ in proper proportions, it has been found possible to produce any color effect, even white. This leads to the conclusion that in the retina of the eye are three different kinds or sets of sensitive nerve endings, sensitive respectively to red, to green, and to blue light. This idea is given corroboration by some facts of color blindness. Thus some persons have no sensation of _red_, this color not being distinguished from green. Others are color blind to green or blue. It is supposed that in color blind persons one of the sets of nerve endings sensitive to one of these three colors is lacking.