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The Colour Box

The colour box is an arrangement for mixing in known proportions the colours from different parts of the spectrum and comparing the compound colour thus produced with some standard colour or with a mixture of colours from some other parts of the spectrum.

Maxwell's colour box is the most complete form of the apparatus, but it is somewhat too complicated for an elementary course of experiments.

We proceed to describe a modification of it, devised by Lord Rayleigh, to mix two spectrum colours together and compare them with a third. This colour box is essentially the spectro-photometer, described in the last section, with the two Nicols F and G removed. Between the lens L and the mirror K is placed a double-image prism of small angle, rendered nearly achromatic for the ordinary rays by means of a glass prism cemented to it. This prism, as well as the mirror K, is capable of adjustment about an axis normal to the bottom of the box. The prism thus forms two images of the slit, the apparent distance between which depends on the angle at which the light falls on the prism; this distance can therefore be varied by turning the prism round its axis.

The light coming from these two images falls on the direct-vision spectroscope SS', and two spectra are thus formed in the focal plane QR. These two spectra overlap, so that at any point, such as B, we have two colours mixed, one from each spectrum. The amount of overlapping and therefore the particular colours which are mixed at each point, depend on the position of the double-image prism, and, by adjusting this, can be varied within certain limits.

Moreover, on passing through the double image prism the light from each slit is polarised, and the planes of polarisation in the two beams are at right angles. We will suppose that the one is horizontal, the other vertical Thus, in the two overlapping spectra the light in one spectrum is polarised horizontally, in the other vertically. For one position of the analysing prism the whole of one spectrum is quenched, for another position at right angles to this the whole of the second spectrum is quenched. The proportion of light, then, which reaches the eye when the two spectra are viewed, depends on the position of the analyser, and can be varied by turning this round. Thus, by rotating the analyser we can obtain the colour formed by the mixture of two spectrum colours in any desired proportions, and at the same time the proportions can be calculated by noting the position of the pointer attached to the analyser. For if we call A and B the two colours, and suppose that when the pointer reads 0° the whole of the light from A and none of that from B passes through, and when it reads 90° all the light from B and none from A is transmitted, while α, β denote the maximum brightnesses of the two as they would reach the eye if the Nicol H were removed, then when the pointer reads θ° we shall have

The standard light will be that in the lower part of the field, which comes from the slit C, after reflexion at the mirror K. This light being almost unpolarised - the reflexions and refractions it undergoes slightly polarise it - is only slightly affected in intensity by the motion of the analyser. By adjusting the tap of the gas-burner we can alter its intensity, and by turning the mirror K we can bring any desired portion of the spectrum to the point B.

The instrument was designed to show that a pure yellow, such as that near the D line, could be matched by a mixture of red and green in proper proportions, and to measure those proportions. It is arranged, therefore, in such a way that the red of one spectrum and the green of the other overlap in the upper half of the field at B, while the yellow of the light from C is visible at the same time in the lower half.

Experiment. - Determine the proportions of red and green light required to match the given yellow.

Enter results thus:

Last Update: 2011-03-27