Colour vision

by Sir William de Wiveleslie Abney

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].

Fig. 4.

R R are rays coming from the source of light, be it sun light or the electric light, and an image of the one or the other is formed by a lens L₁ on the slit S₁ of the collimator C. The parallel rays produced by the lens L₂ are partially refracted and partially reflected. The former pass through the prisms P₁, P₂, and are focussed to form a spectrum at D by a lens L₃. D is a movable screen in which is an aperture S₂, the width of which can be varied as desired. The rays are again collected by a lens L₄, and form a white image of the surface of the last prism on the screen E. If the light passing through S₂ is alone used, the image at E is formed of practically monochromatic light. Part of the rays falling on P₁ are, as just said, reflected, but as it and the refracted part are portions of the light passing through the slit S₁, they both must vary proportionally. If then we use the reflected portion as a comparison light to the spectrum colours, the relative intensities of the two, though they may vary intrinsically, will remain the same. The rays reflected from P₁ fall on G, a silver or glass mirror, and, by means of another lens L₅, also can be caused to form a white patch on the screen E, alongside the patch of colour. At M, or anywhere in the path of the beams, an electro-motor driving a sector with apertures which can be opened or closed whilst rotating, is placed, and the illumination of either beam can be altered at will. To obtain a large spectrum on the screen E, all that is necessary is to interpose a lens of fairly short focus in front of L₄, when a spectrum of great purity and brightness can be formed.

If it be required to measure the width of the slits S₂ (which we shall see further on is often necessary), a small lens of short focal length placed behind L₄ and near the slit will cast a magnified image on E, and by means of a scale placed there, the widths of each slit, if there are more than one, can be read off on the scale by bringing them successively into the same colour.

Fig. 5.

Originally the comparison light was a candle, and it answered its purpose fairly well, and for obtaining absolute measures is convenient at the present time. [Fig. 5] will show its arrangement, but as both the candle and the electric light may vary independently of each other, it will be seen that for merely the comparison of the different spectrum colours, the previous arrangement is the better. In both cases the two beams—the direct and the comparison—may be made to cast shadows by placing a rod in their path, the shadow cast by one light is then illuminated by the other light. By moving the rod towards or from the screen the shadows can be brought side by side.

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.