There is the fact to remember in making these measures in the extreme red and the extreme violet, that the luminosities of the colours are so small that the illumination of the prism itself, by the white light falling on it, has to be taken into account, since it forms an appreciable portion of the patch of feeble colour. By placing a proper shade of blue or red glass in the front of the collimator slit this white light disappears or becomes negligible, and when the absorption of the coloured glass is known from measurement, we can get a very accurate measure of the extinction of these parts. Some people may propound the idea that the rotating sectors may in such kind of measurements give a false result. Now such a criticism is quite fair, and it is absolutely necessary that it should be answered. Well, to test the accuracy or the reverse of the assumption that such measures are correct, the following small piece of simple apparatus was devised. A and B (Fig. 29) are two mirrors placed at angles of 45° to the angle of incidence of the beam. The path the beam takes can be readily ascertained from the figure. This piece of apparatus was placed in position in front of the spectrum, and the reflected beams used to form the patches of colour. For convenience only a small pencil of light was allowed to issue from the prism, a diaphragm of some ½-inch in diameter being placed in front of it. This allows a spot of any desired colour to fall on the screen, the ground glass being removed. The slit through which the spectrum colours pass is moved along the spectrum, and a position is arrived at where the last glimmer of light disappears. [Illustration: FIG. 29.]