A SIMPLE method of obtaining the sensation of yellow produced by the mixture of red and green lights is afforded by a small direct-vision spectroscope of the ordinary kind in which the slit can be rotated to adjust its line perpendicular to the plane, of refraction. If the slit is turned slowly, from this normal position, the bands, of different colours, of course, take up a sloping direction across the spectrum, like books on a half-filled shelf. As the slope increases, the upper end, for example of the red, closes down on the lower end of the green, and as the two blend the clear yellow tint is produced. Other colour mixtures can be similarly noticed.