+----------------------------------------------------+ | | | Luminosity. | | | |-------------------+ | Line. | Colour. | Normal | Red | | | | Eye. | Colour | | | | | Blind. | +---------------+----------------+--------+----------+ | A | Very dark Red | -- | -- | | B | Red (Crimson) | 1·0 | 0 | | Red Lithium | Red (Crimson) | 8·5 | ·5 | | C | Red (Scarlet) | 20·6 | 2·1 | | D | Orange | 98·5 | 53·0 | | E | Green | 50·0 | 49·0 | | F | Blue Green | 7·0 | 7·0 | | Blue Lithium | Blue | 1·9 | 1·9 | | G | Violet | ·6 | ·6 | | H | Faint Lavender | -- | -- | +----------------------------------------------------+ The failure of the red colour-blind person to perceive red is very well shown from this table. It will for instance be noticed that he perceives about one-tenth of the light at C which the normal-eyed person perceives. A modification of this plan can be employed for measuring the luminosity of the spectrum, and it is _excessively_ useful, because we can adapt it to the measurement of colours other than these simple ones. In the plan already explained it was the colour in the patch that was altered, to get an equal luminosity with a certain luminosity of white light. In the modified plan the luminosity of the white light is altered, for the luminosity of the shadow illuminated by the reflected beam can be altered rapidly at will by opening or closing the apertures of the sectors whilst it is rotating. The slit in the slide is placed in the spectrum at any desired point, and the aperture of the sectors altered till equal luminosities are secured. The readings by this plan are very accurate, and give the same results as obtained by the previous method employed. It must be remembered that we have so far dealt with colours which are spectrum colours, and which are intense because they are colours produced by the spectrum of an intensely bright source of light. By an artifice we can deduce from this curve the luminosity curve of the spectrum of any other source of light. If by any means we can compare, _inter se_, the intensity of the same rays in two different sources of light, one being the electric light, we can evidently from the above figure deduce the luminosity curve of the spectrum of the other source of light (see p. 109). We can now show how we can adapt the last method to the measurement of the luminosity of the light reflected from pigments. Fig. 12.--Rectangles of White and Vermilion. Fig. 13.--Arrangement for measuring the Luminosities of Pigments.