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42
THE ADVANCE OF PHOTOGRAPHY
placed behind the slit, a pure spectrum appears upon the opposite wall. If the slit is sufficiently narrow, a series of dark lines may be observed within the spectrum, at right angles to it.
These lines were first seen by Wollaston, but were studied more exactly by the celebrated optician Fraunhofer, and called after him Fraunhofer 's lines.
The lines are always found in the same position, so that they can be considered as natural light lines, upon which the scale of colour is written ; and as the music lines serve for the recognition of the musical notes, so do^/the lines of the spectrum indicate fixed points in the scale
of colour.
If we were to speak of the green of the spectrum, this would be a very vague designation ; whereas b}~ mentioning a line of the spectrum in the green, the part of spectrum is at once Fig 13 characterized. For this
purpose Fraunhofer gave the most characteristic lines names, choosing for this purpose the letters of the alphabet ; a certain line in the red he called A , another in the yellow D, one in the violet H, and another H' '. As the number of lines reaches several thousand, these letters do not suffice to indicate them all. (See fig. 14.)
The lines thus named are found in the spectrum of the sunlight ; the light of other stars commonly shows other lines. The light from artificial sources does not show dark, but bright lines ; a flame coloured yellow with common salt shows, for example, two very characteristic lines in the yellow ; a burning magnesium wire shows several blue and green lines when examined spectroscopically.