CHAPTER III
THE MEANING OF THE SPECTRUM
The Spectrum. Dispersion.—When a narrow beam of white light is transmitted through a prism of glass or of any other transparent substance, it is deflected from its original direction and is at the same time spread out into a small fan of rays instead of remaining a single ray. If a screen is placed in the path of these rays a coloured band is formed on it, the least deflected part of the band being red and the colours ranging from red through orange, yellow, green, blue, and indigo, to violet at the most deflected end of the band. This band of colours is called the spectrum of the white light used, and the spreading out of the rays is called dispersion.
Newton's Experiment.—Newton first discovered this fact with an arrangement like that in Fig. 17.
FIG. 17.
If by any means the fan of coloured rays be combined again into a single beam, white light is reformed, and Newton therefore came to the conclusion that white light was a mixture of the various colours in the spectrum, and that the only function of the prism was to separate the constituents. Of the nature of the constituents Newton had little knowledge, since he had rejected the wave theory, which could alone give the clue.