The rays in the different parts of the spectrum being due to these simple vibratory motions, cannot be decomposed further. And it makes no matter whether we _see_ them as different colours or not, they will always issue at the same angle from the same prism (if the prism be used to form the spectrum), when it is turned to the same angle to the incident light. Milestones are useful along a road to tell us where we are in reference to some central place, and these black lines in the spectrum serve the same end. But they have the advantage over the milestone, for whilst the last will tell us how far we are from, say, York or London, the former tell us our distance from a zero point. We thus have a scale of light of different wave-lengths laid down for us, which we can apply to the study of the sensations stimulated in the eye, and so have the means of instituting a comparison between the colour vision of different eyes. A mixed or composite colour is in a different category, however, to the simple colour, as you will see directly. It is one which may be formed by any number of rays of different wave-lengths falling on the eye. What these rays are we can only tell by analysing the light and referring them to the spectrum. The instrument before you is one which I have used before in this theatre; but as the major part of my experiments have been carried out with it, in case those who are present may not be acquainted with it, it will be necessary to describe it very briefly. The general arrangement of the apparatus is given in the accompanying diagram, Fig. 4. [Illustration: FIG. 4.]