The mirrors A and B may both be of plain glass blackened with smoke on one side, or one may be plain glass and one silvered, or they both may be silvered. This, with the power possessed of altering the aperture of the slit of collimator, puts us in possession of ample means of making our measures. We may also use the ground-glass arrangement and use different diaphragms, which puts a further power of variation in our hands. I may at once state that the resulting measurements fell on the curves, obtained by measurements made with the rotating sectors, a sufficient proof that the sectors may be used with confidence. There is still another method which avoids a resort to the sectors. A tapering wedge of black glass can be moved in front of the colour slit, and a different thickness of glass will be required to cause the extinction of each colour. Recently I have modified the extinction box, more particularly for the purpose of using it where the spectrum is to be formed of a feeble light, such as that of an incandescent lamp or a candle. If a really black wedge could be obtained, this would seem to be the best method, but no glass is really black. We have, therefore, to make a preliminary study of the wedge to ascertain accurately the absorption co-efficients for the different rays, a piece of work which requires a good deal of patience, but which, when done, is always at command. In Fig. 28 two branches of the curves are given at the blue end of the spectrum; one is shown as the extinction for the centre of the eye, and the other of the whole eye. Of course the former observations were made by looking direct at the spot. This may appear a very easy matter, but it is not really so simple as it sounds. It is curious how little control there is over the absolute direction of the eyes when the light has almost disappeared. The axes of the eyes are often directed to quite a different point. When the extinction for the whole eye is made, the readings are really much easier, as then the eye roams where it likes, and a final disappearance is noted. When the eye has once been invested with a roving commission, it is hard to control it. In making these observations it was therefore advisable to have data for the first branch of the curve, before commencing to observe for the later. The main cause of difference between the two branches of the curve is due to the absorption by the yellow spot. It might be thought that with the curves (Fig. 28) before us, we have learnt all we can regarding the extinction of light, but is it so? Surely we ought to know something as to the reduction necessary for extinction of the different parts of the spectrum when they are all of equal luminosities and of ordinary brightness.