Oldal 166 [166]
100 LIGHT AND WATER
necessary to point out that the mere mechanical mix¬
ture of these colours on the palette would produce a
totally different hue to that given by their true mix¬
ture, such as the general tone of the rippled water, as
seen from a little distance, caused by the union of
the several colours reflected.* But the true resultant
tone can be given in the same way by viewing a
' On page 66 we referred to the absorption of rays of certain
definite wave-lengths in passing through coloured substances, such
as red glass. In the same way light passing through two glasses
of different colour undergoes two absorptions. If we look through
a blue and yellow glass placed one over the other we see green.
The blue glass, besides transmitting blue light, allows a certain
amount of green light to pass; the yellow glass also transmits some
green light; but the blue rays are cut off by the yellow glass and
the yellow rays by the blue glass. It is thus only the green rays
which are able to escape the twofold absorption and so reach the ~
eye. Similarly with a mixture of blue and yellow pigments the
resulting colour is green; the greater proportion of the incident
rays penetrates to a small depth below the surface before being re¬
flected, and of these rays all except the green are absorbed by the
combined action of the blue and yellow particles of which the
mixture is composed. |
But blue light and yellow light when mingled do not produce
green, but w/zfe. ‘This can be shown by throwing beams of yellow
and blue light on to the same white surface in a darkened room,
or more simply by spinning a disc coloured partly yellow and partly
blue. "Take two of Maxwell’s cardboard discs,
[ELO each with a radial slit, one of the discs being
b painted with ultramarine and the other with pale
: \ sive) (not orange) chrome yellow. Adjust the discs so
that half of one disc is concealed behind the other
Fig. 28. —as shown in Fig. 28, where a—a indicates the
radius at which the blue disc passes under the
yellow disc, and 4—é the radius where the yellow disc passes
under the blue—on rotating the compound disc, so that the eye