THE ABSORPTION OF LIGHT.
The analysis of light has been explained in a previous chapter, and it
has been shown how the spectrum is produced. Colour, however, may be
obtained by other means, and the property enjoyed by certain bodies, of
absorbing certain coloured rays in preference to others, offers another
mode of decomposing light.
The property of absorption is shown to us in every kind of degree by
innumerable natural and artificial substances; and by examining the
spectrum through a wedge of blue glass, Sir David Brewster was enabled
to separate the seven colours of the spectrum into the three primary
colours, red, yellow, and blue, which he proved existed at every point
of the spectrum, and by over-lapping each other in various proportions,
produce the compound colours of orange, green, indigo, and violet.
Connected with this property is the remarkable effect produced by
coloured light on ordinary colours, and the sickly hue cast upon the
ghost in a melodrama, or the fiery complexion imparted to the hair of
Der Freischutz, or the jaundiced appearance presented by every member of
a juvenile assembly when illuminated with a yellow light from the salt
and burning spirit of "snapdragon," are too well known to require a
lengthened description here.
If a number of colours are painted on cardboard, or groups of plants,
flowers, flags, and shawls, are illuminated by a mono-chromatic light,
and especially the light procured from a large _tow_ torch well supplied
with salt and spirit, the effect is certainly very remarkable; at the
same time it shows how completely substances owe their colour to the
light by which they are illuminated, and it also indicates why ladies
cannot choose colours by candle light, unless of course they propose to
wear the dress only at night, when it is quite prudent to see the
colours in a room lit with gas; and this fact is so well known that with
the chief drapers, such as at Messrs. Halling, Pearce, and Stone's,
Waterloo House, a darkened room lit with gas is provided during the
daytime to enable purchasers of coloured dresses to judge of the effect
of artificial light upon them. Whilst the flowers, &c., are lighted up
with the yellow light, a magical change is brought about by throwing on
suddenly the rays from the oxy-hydrogen light, when the colours are
again restored; or if the latter apparatus is not ready, the combustion
of phosphorus in a jar of oxygen will answer the same purpose. The light
obtained from the combustion of gas affords an excess of the yellow or
red rays of light, which causes the difference between candlelight and
daylight colours already alluded to.
[Page 328]