To the same cause may be traced positive and negative after-images, as they are called. If we look at a strongly-illuminated coloured form, such as a church window, and close the eyes, the window will still be seen, at first of its original colour (a positive after-image), and it will then fade and be seen in its complementary colours (a negative after-image). The positive image is due to the persistence of what we may call nerve irritation, whilst the negative image is due to the physiological excitation of all the nerve fibrils, which ordinarily speaking give the sensation of a very dull white light. The previous fatigue of one set of fibrils, however, prevents them being excited to the same degree as the others, hence we get a complementary image. It would be out of place to pursue this subject further, as we have only dealt with the physical measurement of colour-sensations, and these are beyond it. INDEX. Absorption by red, blue, and green glasses, 53 Absorption of light in the earth's atmosphere, 67 Absorption, reference to law of, 53 After-glow, 74 Arc light, 20 Artists and colours, 195 Balmain's paint, 33 Black body, 18 Blindness to green, 142 Blindness to red, 79-142 Bromo-iodide of silver, 136 Carbon poles, 20 Carmine, light reflected from, 107 " template, 106 Chlorophyll, green solution of, 51 Collimating lens, focal length of, 22 Colour, analysis of, 52 Colour-blind, red and green, 79, 80 Colour-blindness, 142-146, 157, 159 Colour constants, 15 Colour equations, formation of, 147, 148 Colour, extinction of, by white light, 126 Colour mixtures, 113 Colour patch apparatus, 41-52 Colour sensation of the eye, 202 Coloured discs, use of, 189 Coloured glasses, measurement of, 162 Colours, complementary of pigments, 170-172 Colours, complementary of spectrum, 167 Colours, how matched, 156, 157 Complementary colours, measurement of, 173-178 Compound colours, definition of, 16