On the spectroscopic examination of colour produced by simultaneous contrast

In a previous communication I have described some methods of using the spectroscope to analyse sensations of successive contrast. In those experiments the eye, after having been fatigued by monochromatic—preferably spectral—light, is exposed to a second stimulus, consisting also of spectral light, exciting one or more colour-sensations which may or may not include that fatigued by the primary sensation. The question naturally arises, whether the spectroscopic method might not be applied to problems of simultaneous contrast. With this view I made a number of experiments with the Marlborough spectroscope during the summer of 1897, of which the following may be mentioned. A piece of thin cover-glass was fixed in front of the eye-piece at an angle of 45° with the optic axis, so as to reflect into the field of view a small complete spectrum furnished by a 3½ inch direct-vision spectroscope. In order that this might be visible against the bright field of the larger spectroscope, a glass disc, with an opaque spot of the required size painted on it, was inserted in the eye-piece close to the diaphragm.