590 Original Articles. : 7 | Oct., 
ascertaining also how this is modified by the various qualities of those 
bodies which give rise to luminous rays, as well as by the qualities of 
those other bodies whereon they fall. 
If we fasten one end of a long cord to a peg, and holding the 
other end not too tightly in the hand, then strike the cord with a rod, 
we shall perceive that the blow causes an agitation, which travels 
along the cord with a progressive motion. We shall also at once 
comprehend that this something which travels is not a substance but 
a form, and that it is similar in this respect to that appearance which 
sweeps across a field of corn on a windy day. Our readers will 
obtain a very good idea of the undulations which constitute light, if 
they suppose them similar to the waves that travel along such a cord. 
So much for the nature of the light-waves; let us now consider 
their length. Wave-length, or the distance between the crest of one 
wave and that of its neighbour, is a term which explains itself. 
Anyone who has witnessed the phenomena of the ocean, or even of a 
pool of water, can have no difficulty in comprehending what this 
means. If a stone be dropped into a pond, the space between two 
consecutive circles of agitation affords a measure of the wave-length, 
which is, however, very small as compared with the distance between 
two great ocean waves. In the theory of sound the wave-length is an 
important element, and determines the pitch of the note; the rule 
being, that by descending one octave you double the wave-length. 
Now, what have we in optics analogous to pitch in a musical note ? 
Colour will at once be recognized; and we shall all be prepared 
to find that the wave-length of a ray of light determines its colour, 
and that red, orange, yellow, green, blue, violet, &c., have each their 
appropriate wave-length. We need hardly remind our readers that 
a ray of sunlight contains, blended together, not one but many of 
these wave-lengths; for we all know that many colours go to form 
white, and we are no doubt familiar with the method by which a ray 
of white light may be decomposed into its many-coloured components. 
Nevertheless, as this is a subject of very great importance in the 
present inquiry, we may be allowed to discuss it at some length. 
Newton was the first to show that a ray of white light is, in reality, 
compound, and his fundamental experiment may be thus described. 
Let us take a glass prism, and place it in a vertical position. 
Fic. 2. 
Fig. 1 represents an elevation, and Fig. 2 the ground plan of 
