[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 glass 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. End of Project Gutenberg's Curiosities of Light and Sight, by Shelford Bidwell