Note on the sensibility of the eye to variations of wave-length

In a recent communication to the Royal Society, Dr. Edridge-Green has suggested that the reason Lord Rayleigh found he was able to distinguish a difference in hue between two monochromatic patches of yellow (D) light, when they differ in wave-length by about the distance between the sodium lines (0.6 μμ), is that (a) the spectrum used was not pure, and hence the patches were not monochromatic; and (b) that the difference in wave-length was apparent because of admixture with white light. Some experiments made by the author seem so conclusively to show that at any rate the second of the above reasons cannot be correct that it seems worth while to put them on record. By means of Sir William Abney’s double spectrum apparatus, two patches of monochromatic light were thrown side by on a magnesium carbonate screen, and matters were so arranged that no line of separation was observable when the patches were of the same colour. Each patch was 9 mm. by 18 mm., and the observer was at a distance of 60 cm. The intensity of the illumination on the screen was throughout 3.5 candle-metres. The silt in the second spectrum apparatus was kept at a fixed point in the spectrum, while that in the first spectrum was moved by means of a micrometer screw, the movement being read on a scale on which a millimetre represents in the yellow a difference in wave-length of 3.7 μμ.