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FOOTNOTES:

[1] It should be clearly understood that the length of each wave of a series is measured by the distance between the crests of two successive waves. The length of water-waves which break upon a sea sh.o.r.e is not the length along the crest of a single wave measured in a direction parallel to the sh.o.r.e, as the uninitiated are apt to suppose. The true wave-length, or distance from crest to crest of successive waves, can be well observed from the top of a cliff.

[2] In practice, wave-lengths are expressed in ten-millionths of a millimetre. The wave-lengths of the lines A and H of the solar spectrum, which approximately coincide with the limits of visibility, are 7594 and 3968 ten-millionths of a millimetre.

[3] Possibly the human eye is at present in process of transformation from an inferior type to a different and more perfect one.

[4] It is sometimes necessary to place the lens I on the other side of K.

[5] It is easy to find specimens of red and green gla.s.s suitable for this experiment. The proper kind of purple is not so commonly met with.

[6] Some recent experiments on artificial colour-blindness (Proc. Roy.

Soc., Feb., 1898) have led Mr. Burch to the conclusion that there are really _four_ fundamental colour-sensations--a red, a green, a blue, and a violet. His results are, however, thought to be capable of a different interpretation.

[7] Or through several pieces superposed.

[8] A violet-coloured haze may sometimes be actually seen around the opal globes of the electric lamps in the streets.

[9] A "focus" electric lamp was used in the lantern.

[10] Proc. Roy. Soc., Jan., 1899.

[11] After a few seconds' observation the greenish-blue colour often becomes much more intense, but this is an effect of fatigue, with which we are not at present concerned.

[12] See _Nature_, vol. 55, p. 367 (Feb. 18th, 1897).

[13] Or, for best results, use a balanced metal disk covered with black velvet and white paper.