There is one colour, if it may be called so, that has not been shown you, and whether it is a simple colour or not cannot be stated. It seems, however, to be the basis of all other colours, since they all commence with it. It would, perhaps, be preferable to call it the first perception of light instead of a colour. We can exhibit this in a fairly easy manner by a little artifice. An incandescent lamp is before you, and a current from a battery passing through the carbon thread causes it to glow brightly. In the circuit, however, I have introduced what is known as a resistance, which consists of a very large number of square pieces of carbonized linen, pressed more or less tightly together. By means of a screw the pressure can be varied. When the pressure is somewhat relaxed, the resistance to the passage of the current is increased, and the carbon thread glows less brightly; and by a still greater release of pressure, the light can be made to disappear altogether. A beaker (Fig. 10) which we have here is covered with thin blotting paper, and when placed over the incandescent glow-lamp it appears as a luminous yellow cylinder, the colour being due to that of the light within it. We can next insert more resistance in the circuit, and it becomes red, due to the ruddy light of the thread. By inserting still more resistance into the circuit the red fades away, but in the darkness of this lecture theatre the beaker is still a luminous object, though faintly so. It has no colour, and the only sensation it provokes is one of light. Taking off the beaker, we see that the carbon thread is a dull _red_ and nothing more. The passage of this light through the white blotting paper so reduces it that the red is non-existent, and the initial sensation is all we perceive. Placing a piece of red, green, or blue gelatine round the lamp, we get the same effect, showing that the basis of all colour, be it red, green, or any other colour, is what appears to us to be colourless. This experiment is one which is full of interest, as it has a very distinct bearing on diagnosing our colour sensations, and a variation of it will have to be repeated under other conditions. To go back, however, a little way, how does it arise that only three sensations are necessary to give the impression of all colours? One can understand that some definite period of the ether waves might be in unison with the possible swing of one apparatus in the eye, and another with another, but it is somewhat difficult at first sight to conceive that more than one can be made to answer to wave motion of a period with which it is out of tune, so to speak. A couple of illustrations taken from physical experiments may help to suggest how this can happen. [Illustration: FIG. 11.]