(2.) Two solutions of sulphate of copper examined. Standard solution, 10 per cent., 1 cm. in thickness.

Thickness of experimental solution giving the same absorption observed, each mean of five observations.

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 o° the whole of the light from A and none of that from в passes through, and when it reads 90° all the light from B and none from A is transmitted, while a, ß denote the maximum brightnesses of the two as they would reach the eye if the Nicol H were removed, then when the pointer reads 0° 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 itis 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 shew 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.

Captain Abney has recently shewn how, by a modification of Rumford's photometer, the luminous intensity at each point of the spectrum may be compared with that from a given source.

For this purpose a pure spectrum of the given source is produced on a screen. This may be done as in chap. xiv., fig. 34.

It is preferable, however, to use two lenses in such a way that the light from the slit s, (fig. xxxiii), which is placed at the principal focus of the first lens, falls as a parallel beam on the prism P. After refraction through it, parallel rays of each different colour fall on the lens L2, and are brought by it to a focus on the screen D D. In this screen there is a second slit (s), through which rays of only one refrangibility pass. These rays fall on a third lens (L3) arranged so as to produce on a white screen at F E an image of the nearer face of the

prism. This image is illuminated only by light which has passed through s2-that is, by light of a definite colour, and by moving the slit s, a patch of light of any required colour can be thrown on to the screen at F E.

The lenses used will not, in general, be achromatic, and thus the images of s, formed by the different colours will not be at the same distance from L2, but by tilting the screen DD they can all be brought into focus. Again, since the face of the prism P2 is not at right angles to the

direction in which the light travels from it to reach the slit S2, the lens L is also slightly tilted in order to form on F E a sharp image of the whole of this face.

To apply this to colour photometry, a vertical stick is placed in the path of this coloured beam, casting a shadow on the screen, while a second (standard) light (T,), mounted on a scale, casts a second shadow close by. This second shadow is coloured, being illuminated by the coloured beam from S2, while the first shadow receives the light from the standard; still, by moving the comparison light along the scale a point can be found at which the luminosities over the two appear equal. The determination of this point is,

however, attended with some difficulty, much of which is overcome by the adoption of the following oscillation method, the account of which is taken from the Bakerian Lecture for 1886 by Captain Abney and Major General Festing.

The illuminating value of the spectrum varies greatly in its different parts, the maximum usually being in the yellow, and there is a gradation from this towards either end.

Now suppose that with the standard light at, say, 50 cm. from the screen it is approximately of the same intensity as the yellow light of the spectrum, then if the standard be moved to, say, 60 cm. distance there will be two parts of the spectrum, one towards the red the other towards the blue, which will have the same luminosity as the standard at a distance of 60 cm. ; this is, of course, 25/36 of its value when at 50 cm. To find these points, the card to which the slit S, is attached is movable, and the slit can be made to slide along the spectrum, its position being determined by means of a scale. When the standard is at 60 cm. distance and the slit in the yellow, the shadow of the rod illuminated by the white light will be palpably darker than the other; when the slit has passed into the green-blue, it will be palpably lighter. Captain Abney finds that the best way of determining the intermediate point where the shadows balance is by oscillating the slide gently between two points where first one shadow and then the other is palpably too dark; the oscillations become shorter and shorter until the point of balance is determined.' The slide is then moved through the yellow to the red, and the same process is repeated. Two points in the spectrum whose illumination corresponds to that of the standard at the distance of 60 cm. are thus found. This distance is then varied, and another pair of points determined. In this manner a curve is drawn in which the abscissæ represent the position of the slit, while