Joseph von Fraunhofer

"Since the violet rays through the objective of the theodolite telescope have a shorter focal length than the red rays, it is evident why the eye-piece must be displaced in order to see plainly the lines in the different colors."

"In all my experiments I could, owing to lack of time, pay attention to only those matters which appeared to have a bearing upon practical optics. I could either not touch other questions, or at most not follow them very far. Since the path thus traced in optical experiments seems to promise to lead to interesting results, it is greatly to be desired that skilled investigators should devote attention to it."

"Up to the present time, in experiments on diffraction there has been no instrument, except a magnifying-glass, which could be used with profit; and this may perhaps be one of the reasons why in this field of physical optics we are so backward, and why we know so little of the laws of this modification of light."

"The number of different optical phenomena has become in our time so great that caution must be taken so as to avoid being deceived, and also to refer the phenomena to the simple laws."

"In order to receive in the eye all the light diffracted through a narrow opening, and to see the phenomena strongly magnified; still more in order to directly measure the inflection of the light, I placed in front of the objective of a theodolite-telescope a screen in which there was a narrow vertical opening which could be made wider or narrower by means of a screw. By means of a heliostat I threw sunlight into a darkened room through a narrow slit so that it fell upon this screen, through whose opening the light was therefore diffracted. I could then observe through the telescope the phenomena produced by the diffraction, magnified, and yet seen with sufficient brightness; and at the same time I could measure the angles of inflection of the light by means of the theodolite."

"It will reward enough for me if, by the publication of the present experiment, I have directed the attention of investigators to this subject, which still promises much for physicial optics and appears to open a new field."

"I wished to find out whether a similar bright line could be seen in the spectrum of sunlight as in the spectrum of lamplight, and I found, with the telescope, instead of this, an almost countless number of strong and feeble vertical lines which, however, were darker than the other parts of the spectrum, some appearing to be almost perfectly black."

"He was the first to observe spectra due to gratings, and with them he made the earliest determination of wave-lengths."

"Fraunhofer's publication of 1814 did not receive prompt recognition, nor did his papers of 1821 and 1823. Physicists were fighting over the emission and wave theories of light. The attention of chemists was concentrated upon Dalton's atomic theory and the Berthollet-Proust controversy over the law of definite proportions. The full explanation of the new fact brought forth by Fraunhofer was not given for nearly forty years. He himself had failed to find the key to the hieroglyphics of the solar lines, the "Fraunhofer lines," nor had he clearly defined the role which the spectral lines were destined to play in chemical analysis."

"Whether Newton saw the lines or not, he seems to have paid no especial heed to them. In the year 1802, Dr. W. H. Wollaston using, a slit one-twentieth of an inch in width, noted at least four fine dark lines crossing the solar spectrum. Supposing them to be merely 'natural boundaries' of the different colour-bands, he too inquired no further; and there still for a while the matter rested. Nobody yet suspected, even vaguely, what great future results lay enfolded in the casual discovery of these few slight lines. Not many years later the matter was taken up by Fraunhofer, an able German optician."

"He must have been working quietly at the problem through years of European war and tumult. Crowned heads rose and fell; and nations changed hands; and tyrants were cast down; and brave men died by thousands for their countries; whilst Fraunhofer, in the midst of national seethings, calmly investigated the nature of black lines in sunlight."

"With patience he went into the question, using the telescope as well as a very narrow slit... Close examination was rewarded by the making out of lines upon lines; till in the year 1814, that which witnessed the downfall of Napoleon and his banishment to Elba, Fraunhofer had mapped three or four hundred."

"Fraunhofer made a great many experiments connected with these mysterious lines, anxious to discover, if possible, their meaning, For although he now saw the lines, which had scarcely so much as been seen before, he could not understand them; he could not read what they said. They spoke to him, indeed, about the Sun, but they spoke in a foreign language, the key to which he did not possess."

"Fraunhofer had busied himself with glass his entire life. Working with glass was his family tradition, and the manufacture of optical lenses and prisms was his life."

"Fraunhofer's secrets of manufacture accompanied him to the grave. His artisanal knowledge was such that, after his death, even the apprentices who worked with him, in the same glass hut and with the same equipment, achieved only limited success in the manufacture of optical glass."

"By his invention of new and improved methods, machinery, and measuring instruments for grinding and polishing lenses, by his having the superintendence, after 1811, also of the work in glass-melting, enabling him to produce flint and crown glass in larger pieces, free of veins, but especially by his discovery of a method of computing accurately the forms of lenses, he has led practical optics into entirely new paths, and has raised the achromatic telescope to, until then, undreamed of perfection."

"Before we can rightly understand the principles of spectroscopic astronomy, we must go back to the life and work of its founder—Joseph von Fraunhofer. ...Allowing light from the Sun to pass through a prism attached to the telescope, he was amazed to find several dark lines in the spectrum. ...Fraunhofer named the more prominent lines by the letters of the alphabet from A in the red to H in the violet. They are now known as the Fraunhofer lines. ...He expressed the belief that the pair of lines in the solar spectrum which he marked D, coincided with the pair of bright lines emitted by incandescent sodium. Although he doubtless suspected that the lines conveyed intelligence regarding the elements in the Sun, he never was able properly to decipher their meaning. Had he lived he would probably have made the great discovery."

"Fraunhofer discovered that the apparent continuity of a rainbow is an illusion. There are tiny gaps, dim or black arcs of missing colors, too narrow for us to see in the glare of natural rainbows. To say it another way, there are specific colors (specific wavelengths of light) in which sunlight is deficient. Fraunhofer eventually catalogued 576 of these gaps, or "absorption lines": 576 specific wavelengths missing from sunlight. Fraunhofer's career of discovery was cut short by consumption."