With this instrument it is easy to demonstrate that a mixed colour may be mistaken for a simple colour of the spectrum. In a glass cell with parallel sides is a solution of potassium bichromate, which, to myself and probably most of you, has a beautiful orange colour. The spectrum of white light is now on the screen, and if this orange liquid is placed in the path of the white light before it reaches the prisms, all the violet, blue, and most of the green is cut off, leaving some green-yellow, orange and red only on the screen. That these form the orange colour of the bichromate is readily shown by removing the auxiliary lens. The spectrum, which has its focus at D, is now recombined into a patch of light, which is at once seen to be the colour of the solution. [Illustration: FIG. 6.] The colour of the bichromate is therefore a complex or mixed colour according to our definition, for it is made up of a large number of simple colours. What I desire to show, however, is that this complex colour can be mistaken by the eye for a simple colour. First, let us interpose the cell with the bichromate in the path of the _reflected_ beam, and throw the patch of light formed by it on a white surface A (Fig. 6), alongside the patch of light B formed by the spectrum. Next let us pass a single aperture (Fig. 7), which can be opened and closed by a screw arrangement, through the spectrum. By careful movement we at length come to an orange ray, which is spread out by the apparatus to form a patch on B, that to the majority (and the word majority is used with intention) of people exactly matches the colour of the bichromate. Thus we have a proof that, as far as the eye is concerned, the simple and the complex colours are identical. This illustration of the want of power of the eye to analyse colour might be repeated as often as we like. We may pass coloured wools, for instance, through the length of the spectrum and show that they have the property of appearing bright in, and therefore of reflecting, some colours and of almost disappearing in others--a sure indication that these colours are mixed colours as they are made up of the rays which are reflected. Yet when viewed in white light they can in many cases be matched with simple colours in the way we matched the colour of the bichromate solution. This tells us that there is something which requires investigating as to the constitution of the perceiving apparatus, and points to the probability that it is less complicated than it would be were it able to differentiate, without the aid of the spectrum, between simple and complex colours. If the eye had a separate apparatus--and when I say apparatus I use the word for want of a better--for taking up the impression of every simple colour, it might well be assumed that a differentiation must take place.