are said to exhibit _normal dispersive power_. There are, however, some bodies, such as iodine or an alcoholic solution of fuschine, which exhibit _anomalous dispersion_ and refract some longer waves of light more than some shorter ones. The arrangements for forming a normal spectrum are as follows: We pass a beam of light from the electric lamp through a lens, and place in front of this lens a metal plate with a narrow vertical slit-shaped opening in it. At a proper distance in front of the slit we place another lens, and project upon the screen a sharp image of the slit in the shape of a bar of white light. Placing a hollow glass prism filled with bisulphide of carbon in front of the last lens, we find that the various rays in the white light are dispersed, and we produce on the screen a band of rainbow-coloured light, called the spectrum. This spectrum is in reality a series of differently coloured images of the slit placed side by side. By making use of the principle of interference as disclosed by Young, it is possible to make a measurement of the wave-length of the rays of light which produce the sensation of various colours when they fall upon the eye. Thus the wave-length of those æther waves which produce the sensation of deep red is 0·75μ, and that of the waves producing the sensation of violet when they fall upon the retina of the eye is 0·43μ. The whole of the visible spectrum is therefore included within a single octave of æther radiation. Within these limits any change in the wave-lengths makes itself felt in our eyes as a change of colour. It is commonly said that there are seven colours in the spectrum—red, orange, yellow, green, blue, indigo, and violet. As a matter of fact, a highly trained eye can discover about a thousand different tints in the spectrum of white light. Time will not allow us to enter into any discussion of what is called colour-vision and the theory of sensations of colour. The fact I wish to impress upon you here is that, outside of ourselves, there is no such thing as colour. The rays of light which produce these sensations of colour when they enter the eye differ from one another only in wave-length and wave-amplitude. Hence there is a complete analogy between light of different colours and sounds of different pitches or tone. Red light differs from blue light only as a bass note in music differs from a treble note. Hence you must distinguish very carefully between a ray of light in itself, and the sensation it produces when it falls upon the retina of the eye. Our eyes are gifted with a marvellous power of detecting slight differences between the wave-length and the amplitude of the rays which may stimulate two adjacent portions of the retina of our eyes.