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. But this is not all with a dyed fabric, for
here the light is not simply reflected light; part of it has traversed
the upper layers of that coloured body, and is then reflected from the
interior, losing a portion of its coloured rays by absorption. This
reflected coloured light is always mixed with a certain amount of white
light reflected from the actual surface of the body before penetrating
its uppermost layer. Thus, if dyed fabrics are examined by the
spectroscope, the same appearances are generally observed as with the
solution of the corresponding colouring matters. An absorption spectrum
is in each case obtained, but the one from the solution is the purer,
for it does not contain the mixed white light reflected from the
surfaces of coloured objects. Let us now take an example. We will take a
cylinder glass full of picric acid in water, and of a yellow colour. Now
when I pass white light through that solution and examine the emerging
light, which looks, to my naked eye, yellow, I find by the spectroscope
that what has taken place is this: the blue part of the spectrum is
totally extinguished as far as G and 2/3 of F. That is all. Then why,
say you, does that liquid look yellow if all the rest of those rays pass
through and enter the eye, namely, the blue-green with a trifle of blue,
the green, yellow, orange, and