roduce white, but if we mix a yellow and a
blue paint or dye we have as the result a green colour. How is this?"
The cases are entirely different, as I shall proceed to show. In
speaking of the first, the complementary colours, we speak of pure
spectral colours, coloured rays of light; in the latter, of pigment or
dye colours. As we shall see, in the first, we have an addition direct
of coloured lights producing white; in the latter, the green colour,
appearing as the result of the mixture of the blue and yellow pigments,
is obtained by the subtraction of colours; it is due to the absorption,
by the blue and yellow pigments, of all the spectrum, practically,
except the green portion. In the case of coloured objects, we are then
confronted with the fact that these objects appear coloured because of
an absorption by the colouring matter of every part of the rays of light
falling thereupon, except that of the colour of the object, which colour
is thrown off or reflected. This will appear clearer as we proceed. Now
let me point out a further fact and indicate another step which will
show you the value of such knowledge as this if properly applied. I said
that if we selected from the coloured light spectrum, separated from
white light by a prism, say, the orange portion, and boring a hole in
our screen, if we caught that orange light in another prism, it would
emerge as orange light, and suffer no further analysis. It cannot be
resolved into red and yellow, as some might have supposed, it is
monochromatic light, _i.e._ light purely of one colour. But when a
mixture of red and yellow light, which means, of course, a mixture of
rays of greater and less refrangibility respectively than our spectral
orange, the monochromatic orange--is allowed to strike the eye, then we
have again the impression of orange. How are we to distinguish a pure
and monochromatic orange colour from a colour produced by a mixture of
red and yellow? In short, how are we to distinguish whether colours are
homogeneous or mixed? The answer is, that this can only be done by the
prism, apart from chemical analysis or testing of the substances.

[Illustration: FIG. 16.]

The spectroscope is a convenient prism-arrangement, such that the
analytical effect produced by that prism is looked at through a
telescope, and the light that falls on the prism is carefully preserved
from other light by passing it along a tube after only admitting a small
quantity through a