, Fig. 16). We speak, for
example, of the soda yellow-line as coinciding with D of the spectrum.
These, then, are spectra produced by luminous bodies.
The colouring matters and dyes, their solutions, and the substances dyed
with them, are not, of course, luminous, but they do convert white light
which strikes upon or traverses them into coloured light, and that is
why they, in fact, appear either as coloured substances or solutions.
The explanation of the coloured appearance is that the coloured
substances or solutions have the power to absorb from the white light
that strikes or traverses them, all the rays of the spectrum but those
which are of the colour of the substance or solution in question, these
latter being thrown off or reflected, and so striking the eye of the
observer. Take a solution of Magenta, for example, and place a light
behind it. All the rays of that white light are absorbed except the red
ones, which pass through and are seen. Thus the liquid appears red. If a
dyed piece be taken, the light strikes it, and if a pure red, from that
light all the rays but red are absorbed, and so red light alone is
reflected from its surface.
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