CHAPTER VI.

DISPERSION AND ACHROMATISM.

HITHERTO We have considered light to be simple or homogeneous. The light of the sun, however, is not homogeneous but compound; each ray of solar light is composed of an infinite number of rays of homogeneous light differing from each other in colour and refrangibility. This fact was first established by Newton.

In Newton's first experiments his room was darkened and a beam of the sun's light admitted through a small circular hole in the shutter of one of the windows. This beam of light made a small circular spot of white light on the opposite wall. He then placed a triangular prism of glass near the hole, with its edge downwards and perpendicular to the beam of sunlight, so that the rays passed through the prism close to its edge. The patch of light on the wall was no longer circular and white, but elongated and coloured with vivid and intense colours. The sides of the coloured image or spectrum were both straight and perpendicular to the edge of the prism, and the ends appeared semi-circular. The breadth of the spectrum was the same as that of the circular white spot, while its length was about five times greater.

This elongation of the image can only be explained by supposing that the rays of the beam of sunlight are refrangible in different degrees. The rays from the sun are not quite parallel, for some might proceed from the upper and others from the lower limb of the sun's disc. But when the prism is placed in its position of minimum deviation, a small difference of incidence will produce no

appreciable difference of deviation; consequently the inclination of the emergent rays will be the same as those of the incident rays; and therefore if the beam of light were homogeneous it would cause a circular spot of white light of the same dimensions as before, but in a displaced position.

This experiment further shows that those rays which differ in refrangibility differ also in colour; for the coloured spectrum is red at its lower or least refracted end, and the colour changes by imperceptible gradations through yellow, green, blue, until at the upper or most refracted end it is violet. Newton distinguished seven principal colours; these arranged in order of their refrangibility are red, orange, yellow, green, blue, indigo, violet. Of these the orange and yellow are the most luminous, the red and green next in order, and the indigo and violet weakest.

85. After trying several ways of explaining those phenomena Newton was finally led to the following experimentum crucis, which is described almost in Newton's own words. He took two boards, and placed one of them close behind the prism at the window, so that the light might pass through a small hole, made in it for the purpose, and fall on the other board, which was placed at about twelve feet distance, a small hole having first been made in it also for some of that incident light to pass through. Then he placed another prism behind this second board, so that the light passing through the two boards might pass through that also, and be again refracted before it reached the wall. This done, he took the first prism and turned it slowly to and fro about its axis, so as to make the several parts of the image cast on the second board successively pass through the hole in it, and observed to what places on the wall they were refracted by the second prism. He saw that the light tending towards the violet end of the spectrum was considerably more refracted than the light tending towards the red end. Hence he concluded that sunlight is not homogeneous, but consists of rays of different colours, some of which are more refrangible than others.

86. In this form of the experiment the different coloured images of the sun are of considerable size, and are arranged with their centres along a straight line. The coloured images will therefore overlap, and the colours will not be thoroughly separated; the spectrum is then said to be impure. We shall now show how a pure spectrum may be obtained.

The sun is always moving relatively to the earth and therefore the direction of his rays is continually changing. This change of direction may be corrected by an instrument called a heliostat, which consists of a mirror turned by clockwork in such a way that the light is always reflected in the same direction. The reflected rays of the sun are allowed to fall on a convex lens of short focal length, so as to make a very small image of the sun at the focus of the lens; this image may easily be made so small that it may be regarded as a point. A small pencil may be selected from the rays passing through this point by making them fall on a very narrow slit between two carefully worked plates of metal. If a cylindrical lens with its generating lines parallel to the slit be used, the rays may be concentrated on the slit throughout its whole length, and a very bright thin pencil can be obtained. The pencil of light is allowed to fall on a prism near the refracting edge, this edge being parallel to the slit. The prism must be placed in the position of minimum deviation for rays of mean refrangibility, and then it will be nearly in a position of minimum deviation for all rays.

Let be the small focus or the section of the slit through which the rays pass. Then after refraction at the prism the red rays will diverge from a point r, and the violet rays from a point v, where Av = Ar=AQ. If the colours be received on a screen, they will overlap, and though by moving the screen farther away from the edge of the prism, the colours become more and more separated, yet they become fainter at the same time. The pencil is therefore made to pass through an achromatic lens (the construction of which will be hereafter described), whose centre is B, after which the red rays

will converge to a focus r', and the violet rays to a focus

B

v', where rBr', vBv' are straight lines. The colours are now perfectly separated, but the spectrum v'r' is very small, so that it needs to be magnified before it can be accurately measured. The spectrum is therefore viewed through another lens or eye-piece (also corrected for chromatic dispersion). The two lenses constitute an ordinary astronomical telescope. If therefore the rays from the prism be received on a telescope, by focusing the telescope we shall be able to see a pure spectrum.

If we wish to exhibit the spectrum on a screen, the lens must be removed. In this case it is better to put between Q and A a lens whose focus is at Q. Then the rays after passing this lens are parallel and the points v and r are at an infinite distance; and by moving the screen further from A we separate the colours more and more without weakening their intensity.

Newton himself described fully how a pure spectrum might be obtained by the use of a lens and a narrow slit in front of the prism.

87. If a pure solar spectrum be examined carefully, it is found that it is not a continuous coloured band, but that there are at certain intervals abrupt deficiencies of light, forming dark lines across the spectrum. These lines are always seen irregularly disposed along the spectrum whatever refracting substance may be used. When the refracting substance is varied, the positions of the lines change, but they and the coloured rays always appear in

the same order, so that any line can be recognised. As these lines are sharp and definite and are always present, they can be used as marks for determining refractive indices; the refractive indices of the rays to which they correspond can be determined for any substance with an accuracy equal to that of astronomical measurements. The positions of these lines, to the number of seven hundred, have been carefully measured and mapped out by Fraunhofer and others, and the refractive indices of the corresponding rays accurately determined for a very large number of substances. By using prisms of the same substance but of different refracting angles, Fraunhofer verified the law of refraction for the rays corresponding to any one of the fixed lines, with extreme accuracy. These dark lines are not characteristic of light in general, but only of solar light; for if the slit be illuminated by a gasflame, a perfectly continuous spectrum is observed.

The brightness of the solar spectrum is by no means uniform; it is brightest in the yellow and the neighbouring colours, orange and light green, and falls off gradually on both sides. It may be observed here, though this scarcely belongs to the province of optics, that the solar rays as separated into a spectrum differ from each other also in heating and chemical effects. The heating effect increases as we pass from the violet to the red rays, and still continues to increase for a certain distance beyond the visible spectrum, at the red end. Similarly, if the action of the different rays on a sheet of sensitive paper be observed, the action is very feeble in the red, strong in the blue and violet, and is sensible to a great distance beyond the violet end of the spectrum.

88. There are three different kinds of spectra depending upon the nature of the source of the light employed.

i. The solar spectrum is a continuous spectrum, except that it is interrupted by a definite system of dark lines. The spectra of fixed stars also contain dark lines, different for different stars.