Those that do comprise the visible spectrum and the different
wave-lengths of visible radiant energy manifest themselves by arousing
the sensations of the various spectral colors. The radiant energy of
shortest wave-length perceptible by the visual apparatus excites the
sensation of violet and the longest ones the sensation of deep red.
Between these two extremes of the visible spectrum, the chief spectral
colors are blue, green, yellow, orange, and red in the order of
increasing wave-lengths. Electromagnetic energy radiated by a
light-source in waves of too great wave-length to be perceived by the
eye as light is termed as a class "infra-red radiant energy." Those too
short to be perceived as light are termed as a class "ultraviolet
radiant energy." A solid body at a high temperature emits
electro-magnetic energy of all wave-lengths, from the shortest
ultra-violet to the longest infra-red.

Another complication arises in the variation in visibility or luminosity
of energy of wave-lengths within the range of the visible spectrum.
Obviously, no amount of energy incapable of exciting the sensation of
light will be visible. The energy of those wave-lengths near the ends
of the visible spectrum will be inefficient in producing light. That
energy which excites the sensation of yellow-green produces the greatest
luminosity per unit of energy and is the most efficient light. The
visibility or luminous efficiency of radiant energy may be ranged
approximately in this manner according to the colors aroused:
yellow-green, yellow, green, orange, blue-green, red, blue, deep red,
violet.

Newton, an English scientist, first described the discovery of the
visible spectrum and this is of such fundamental importance in the
science of light that the first paragraph of his original paper in the
"Transactions of the Royal Society of London" is quoted as follows:

In the Year 1666 (at which time I applied my self to the
Grinding of Optick Glasses of other Figures than Spherical) I
procured me a Triangular Glass-Prism, to try therewith the
celebrated Phaenomena of Colours. And in order thereto, having
darkened my Chamber, and made a small Hole in my Window-Shuts,
to let in a convenient Quantity of the Sun's Light, I placed my
Prism at its Entrance, that it might be thereby refracted to
the opposite Wall. It was at first a very pleasing
Divertisement, to view the vivid and intense Colours produ