SPECTROSCOPE.

Since the invention of the telescope no discovery has given so great
an impetus to astronomical physics as the spectroscope; and in giving
us information about the systems of stars and their proper motions it
rivals the telescope.

Frauenhofer, at the beginning of the nineteenth century, while
applying Dollond's discovery to make large achromatic telescopes,
studied the dispersion of light by a prism. Admitting the light of the
sun through a narrow slit in a window-shutter, an inverted image of
the slit can be thrown, by a lens of suitable focal length, on the
wall opposite. If a wedge or prism of glass be interposed, the image
is deflected to one side; but, as Newton had shown, the images formed
by the different colours of which white light is composed are
deflected to different extents--the violet most, the red least. The
number of colours forming images is so numerous as to form a
continuous spectrum on the wall with all the colours--red, orange,
yellow, green, blue, indigo, and violet. But Frauenhofer found with a
narrow slit, well focussed by the lens, that some colours were missing
in the white light of the sun, and these were shown by dark lines
across the spectrum. These are the Frauenhofer lines, some of which
he named by the letters of the alphabet. The D line is a very marked
one in the yellow. These dark lines in the solar spectrum had already
been observed by Wollaston. [3]

On examining artificial lights it was found that incandescent solids
and liquids (including the carbon glowing in a white gas flame) give
continuous spectra; gases, except under enormous pressure, give bright
lines. If sodium or common salt be thrown on the colourless flame of a