Page:Popular Science Monthly Volume 6.djvu/761

Rh at Munich. Four years before his death, this Academy appointed him keeper of its Museum of Physics. He had the Order of Civil Merit conferred on him by the King of Bavaria, and received the Order of Danebrog from the King of Denmark.

It was as a scientific optician, not only thoroughly familiar with the theory of the subject, but skillful in the use of instruments, and an accurate and painstaking observer, that Fraunhofer entered upon the exploration of a new phenomenon in the solar spectrum.

It is well known that we are indebted to Sir Isaac Newton for the capital experiment of the decomposition of white light into its constituent color-rays. He passed the beam from an opening in a shutter through a glass prism in a darkened room, and got the image of colors in the order of their refrangibility, forming what is familiarly known as the solar spectrum. But this spectrum is not pure. He used the light from a round hole in the shutter, and the ray, when passing through the prism, gave a series of overlapping images of the aperture, by which the colors of the spectrum were somewhat mixed, and, in consequence of this, there was a peculiar class of effects which he did not recognize. His experiment was made in the year 1675, exactly 200 years ago, and for 127 years his method of forming the spectrum was followed, and no step was taken toward the discovery of the phenomena now to be considered. But, in 1802, Dr. Wollaston, an Englishman, examined the spectrum formed by a narrow opening or slit, and found that instead of being so pure as had always been supposed, it was crossed in various places by fine dark lines. The observation attracted no attention at the time, and was not followed up by himself or others.

These dark lines were afterward rediscovered by Fraunhofer, who became so much interested in them that he made them the subject of careful investigation, and his results were so accurate and complete as to have been universally accepted when published in 1814, and the lines from that time have gone under the name of "Fraunhofer's lines." By means of a telescope he observed the spectrum formed by a fine slit, and found that it was crowded with these fine dark lines; that they varied somewhat in thickness, and were distributed in unequal groups throughout the spectral space. He counted 590, from the red to the violet, and made an accurate map of them, as represented in the preceding figure, designating the most important by the letters of the alphabet, large and small, which are still constantly used in the investigations of spectrum analysis.

To the question, What are these dark lines? no clear answer could be given. Science was not as yet prepared to offer an explanation of their cause. Yet Fraunhofer's mind was not idle in regard to this point, and he speculated with great sagacity in the right direction. Optically, or with reference to figure, the dark lines are simply images of the slit. There are of course no such lines in sunlight, but there is