Popular Science Monthly/Volume 6/April 1875/Sketch of Dr. Joseph Fraunhofer
|SKETCH OF DR. JOSEPH FRAUNHOFER.|
FEW things in the history of science are more interesting than the examples it affords of men devoting themselves with passionate assiduity and untiring persistence to researches which the investigator himself can neither turn to account nor are to be of any ultimate use, and which the public regards as in the last degree frivolous and futile, but the value of which is ultimately and abundantly justified. A man works like a martyr at some obscure and unknown subject, is laughed at and commiserated by his contemporaries, dies, and drops out of memory, until in after-times, in the turns of thought, his results are suddenly invested with a grand interest, and become a passport to their author's immortality. Something like this was the fortune of the subject of the present sketch; he worked in a field which nobody thought of the slightest moment, but he has linked his name forever with one of the most brilliant discoveries of this century.
Joseph von Fraunhofer was born in Straubing, Bavaria, March 6, 1787, of humble parents, and was left an orphan in 1799, at the age of eleven years, when he was bound out as apprentice to a glass-cutter in Munich, who was very exacting of him in the way and time of his work.
|FRAUNHOFER'S LINES IN THE SOLAR SPECTRUM.|
The young apprentice, however, without any instructor, found means, in what little time he had to himself, to make up as well as he could the deficiencies of his education, and made great progress in mathematics, besides getting a knowledge of astronomy. From this station he owed his elevation to an accident when nineteen years old. Working himself out unhurt from an old tumble-down house which fell in on him, he became a sort of neighborhood hero, attracting the attention of some gentlemen of wealth and rank who took an interest in the apprentice whom they found trying to cultivate the sciences in such adverse circumstances. They brought him to the notice of and introduced him to the celebrated Reichenbach, who gave him a place as optician in his great establishment for the construction of mathematical and philosophical instruments at Benedictbaiern, near Munich. He found here ample scope for the exercise of his talents, and could now study optics as a science. He soon distinguished himself by his inventive genius and the skill he displayed in the execution of the processes which he was employed upon. The advantages which he enjoyed he turned to account in making many important experiments in light, and constructing superior instruments for astronomical observations. His discoveries greatly increased the reputation of the establishment, of which he finally became the proprietor himself. He made the best crown-glass for achromatic optical instruments, and invented a heliometer, micrometer, and many other scientific contrivances. The celebrated equatorial telescope in the Russian observatory at Dorpat was made by him. In 1819 he moved his great establishment to Munich, and continued his work there till his death, which occurred June 7, 1826, soon after entering upon the fortieth year of his age. He was a member of the University of Erlangen, and of the Royal Academy of Sciences 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 in sunlight that which takes the shape of lines when passed through a narrow opening. The white image from such an opening would of course be a fine white line. If passed through a prism the white line would become a series of colored lines, each point or ray of which would be an image of the slit. If the slit is changed to a cross, then there are crosses in the spectrum. The dark lines, of course, are lines in which light is absent, for darkness is absence of light. There are then certain missing rays in the sunlight that come out in the spectrum as lines of vacancy or breaks in its continuity, and Fraunhofer's lines are all of this kind.
Now, Fraunhofer made the important discovery that the lines of sunlight did not vary when examined at different times. His 590 lines were there, in their exact places, at all times of the day, and at all seasons of the year; the cause was therefore probably not in the earth's atmosphere; did not pertain to the earth, and therefore probably existed in the sun. Furthermore, he found that the light from the moon and from Venus gave the same system of dark lines. Fraunhofer saw in Venus-light the double D lines; b also was seen double, and the relative distance from D to E, and from E to F, was the same in the Venus as in the solar spectrum. As this light is reflected from the sun, Fraunhofer was confirmed in the conclusion that these lines are of solar origin.
But he went still further. He made careful and extensive observations of the spectra from the fixed stars, and made the striking discovery that they give groupings of dark lines, which differ from those of the sun and from each other. Some of the stellar lines, however, he showed to be identical with those of the sun. Among the lines of the bright star Procyon he recognized the solar line D; and in those of Capella and Betelgueux he found both D and b. Fraunhofer made also the important observation that the bright-yellow line characteristic of the spectrum of sodium exactly coincided with the double dark solar line D. But he could not take a step toward explaining the connection. It was impossible for him to know in what way special rays were cut out or absorbed in the sun and stars, so as to give only darkness, but he got clearly before him the great problem which it is the glory of spectrum analysis to have since resolved.
- Pronounced Frown'-ho-fer.