FRAUNHOFER LINES.
The spectrum given by sunlight is not a continuous one. It is marked by fine black lines the whole of the way the red to the violet. These lines, first noticed by Wollaston, are due to absorp tion. The number and distinctness of them depends on the resolving power of the observing telescope and also on the ' width of the slit. The immense number of these lines makes the Fratinhofer spectrum very valuable for comparison purposes with the lines of other spectra. The chief lines are denoted by the first eight letters of the alphabet. They afford a rough method of dividing the spectrum into regions, but the most satisfactory way to refer to them is by stating their wave lengths. These dark lines indicate that the radiation given out by the luminous part of the sun, the photosphere, is par tially absorbed by a cooler layer of the same illuminating substances surrounding it, called the reversing layer. This effect can be shown very simply as follows. ? Use an arc light giving a continuous I spectrum, and interpose a sodium flame between the arc and the slit. The incan
descent sodium vapour alone gives two bright yellow lines very close together and ll,), but under the circumstances just mentioned a bright spectrum will be seen crossed by two vertical dark lines where the two yellow lines were seen before. The Bunsen flame is cooler than the arc crater, and, as a result, it absorbs energy in the form of the radiation it emits itself, from the more intense light given out by the are. Matter always ab sorbs those radiations most easily which it gives out itself when excited and snowed to vibrate freely. The absorbed vibrations will be of greater amplitude than those of the absorbing particle, i.e. the source giving out the vibrations will be hotter than the body absorbing them, for increased activity of molecular motion means increase of temperature.
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