XCV
produce red colour and rays shorter than such, which are seen
in the violet colour cannot be observed, and are called dark rays
of light. Of the existence of such rays, we know by the thermal
and chemical effects of light. Heat increases from violet to
wards red and is the strongest beyond red, where we see noth
ing. The chemical effects are in blue and violet stronger than
in other colours and strongest beyond the violet colour. This
shows, that on both sides of the spectrum there exist rays of
light, which we cannot observe. The longest light-waves (red)
are refracted least, the shortest (violet) strongest. Some kinds
of fluor-spar (fluor-calcium) show a peculiar shining splendour,
which is called fluorescence. Phosphate of uranium, petroleum,
sulphate of quinine, a solution of spirit of wine and cloro
phyle etc. show the same property. What is most striking
in the matter is the fact, that these bodies show colours which
are not contained in the light falling upon them, hence it seems
as if these substances were capable of changing the colours, which
means they change the refraction of the rays and the lengths of
the waves of light, so that dark rays of light are rendered visible.
If we make rays of light collected by a lens pass through a so
lution of copper, which allows only blade rays to pass through,
agree coloured cone will be seen in phosphate of uranium.
§ 232. Optical phenomena in the atmosphere.
1) The blue colour of the sky. 406. The atmosphere is
not perfectly transparent and reflects especially blue light (else
the sky would be black. Aeronauts). Vapours make its colour
whiter, hence in winter paler than in summer; in the southern
hemisphere bluer, why?
2) Morning red a evening red, 407, due to the va
pours which in their liquifying state reflect orange rays (vapour
streaming forth from a locomotive).
