XCIII
be reproduced. 396. Newton made a disc, the centre and the
edges of which are covered with black paper, the space between
consists of papers of the colours of the spectrum. When this
disc is rapidly rotated it appears white. Also the luminous
power of the various spectral rays differs, the yellow and green
being the most powerful rays. If the spectrum is caught up by
a screen with a hole in the middle in such a manner that only
green or red or blue rays can pass through the hole and these
rays fall upon a second prism, they will not be decomposed
again, thus proving that the colours of the spectrum are single
and are not a mixture of other colours.
§ 226. In the middle of the spectrum a great number of
dark lines have been discovered, Fraunhofer, a German optician,
described them first, and so they are called Fraunhofer's lines.
In the case of the sun's light these lines are always fixed and
definite, in flames containing chemical substances these lines
replaced by shining lines differ. By Kirchhoff and Bunsen this
fact was made a basis for detecting minute portions of any
substance, in observing that salts contained in the flame of metals
produce always lines identical in colour and position. The
method, according to which chemical substances are detected by
means of these lines and the colours of the spectrum, is called
the spectrum analysis, the instrument used for such investi
gations is known as Spectroscope. By this method some quite
new substances (Calsium, Rubidium, Thallium, Indium) have
been discovered. The difficulty to avoid the lines produced by
Sodium, f.e., shows that Sodium is contained almost in every
thing. For vapourizing the different alkalies and metals
electric sparks are applied. It is very likely, that Fraun
hofer's lines are caused by various metallic vapours contained
in the sun's atmosphere, these vapours absorb the rays of cer
