XCIV
makes for some time the eye insusceptible to this colour and
the complementary colour takes its place. All this shows that
colour is not an essential property of the objects themselves,
but the product of a peculiar sensation caused by the irritation
of our optical organ. Hence even without light by a mere agi
tation of the visual nerve the sensation of colours may be pro
duced (by a thrust against our eye, by an electric stream).
§ 229. The Jesuit Grimaldi discovered in the 17th century,
that rays passing through a small aperture in a dark room, the
spot light shines upon, is larger than we would have expected
from the rays coming in. This proves that light, in passing the
edges of opaque bodies is turned off and diverges. This turn
ing off is called the Inflection of light. 408. The edges of the
spot lighted upon are in general coloured.
§ 230. The phenomena of Interference (see § 118) may
be accompanied by beautiful colours. If rays of light fall upon
thin leaves of a transparent body, it may be, that light reflected
by the front-side of the leaf will interfere with such light which
after having been reflected by the back-side of the leaf and twice
refracted comes back again. By this inflexion and interference
the splendour of the mother of pearl, 409; and the beautiful
colours of soap-bubbles, 410, are produced. Also the colours
of an old window-glass and of stagnant waters may be explain
ed this way.
§ 231. Fluorescence. In § 223 we said that the distinc
tion of different colours is due to the different length of the
light-waves just as different tones come to existence by different
lengths of the sound-waves. And in the same manner, in which
we can only perceive sound-waves within a certain limit (see §
108–110), so also the sound-waves can be observed only if they
are not too long or too short. The rays longer than those which
