CHAPTER IV. DISPERSION AND THE SPECTRUM. DIFFERENT COLOURS HAVE DIFFERENT REFRANGIBILITY. The Spectrum-Different Colours differently Refracted-and each Colour has its own Angle of Total Reflection—Position of the Prism and its Effect-Correction of Aberrations by Variations in Position -White Light a Compound of Various Colours-Suppression of Colour produces Colour-Artificial Composition of White Light— A Narrow Slit necessary for a Pure Spectrum-The RainbowRefraction and Dispersion not Proportional-Achromatic Prisms and Lenses--Direct Vision Prisms-Anomalous Dispersion. 41. The Spectrum.-With the water prism as before described, or if the multiplying-glass or lustre-button used to produce the multiple deflected images has faces of small obliquity, nothing more may have been noticed than the refraction described in the last chapter; though even with these instruments attentive observation will generally discover a slight fringe of colour at the edges of the refracted images. We must now, however, employ a prism of more density and greater angle-either the glass prism (Fig. 9), or the prism bottle filled with bisulphide of carbon. Place in the optical slide-stage a perpendicular slit inch deep and inch wide, and arrange either the flint-glass prism, or the bisulphide prism on the stand, as in Fig. 49, first F focusing the slit upon the screen; and, as we expect the rays to be now very seriously deflected, turning the lantern off at a considerable angle before interposing the prism. What a spectacle we have! There stands the glorious rainbow-band as first revealed to Newton's enraptured eyes; and which is to introduce us to a new and magnificent field-that of colour. Of all the people who have experimentally studied Optics, and who of course have performed this experiment scores and scores of times, never one yet but has felt that it never loses its fascination; the same feeling of delight ever comes upon us, as that SPECTRUM appears on the screen (Plate II. A), which is to go with us, and be more or less, our guide, through great part of our future experiments. It is at once noticed, that while all the colours are bent aside, the red end of the spectrum is much less bent than the blue. 42. Different Colours differently Refracted.Newton deduced from this and other experiments with the 1 With a gas-burner the screen distance should not exceed about six feet. spectrum, that each colour of light had its own degree of refrangibility, and that white light was compounded of various colours. We demonstrate the first point as follows. Arrange as before, but with a short as well as narrow slit in the optical stage let it, say, be an aperture inch square--and let its long narrow spectrum be projected by the bisulphide prism. We may perhaps think that the effect of the prism is merely of itself to spread or open the rays. To see if this is the case or not, we adjust behind the first prism, or between it and the screen, our glass prism in a horizontal position, with the refracting edge downward. If it be so, our spectrum will now be generally widened as well as refracted upwards. It is not, however; the violet end is refracted up far more than the red end, and the spectrum appears on the screen askew, or slanting. The spectrum may be perhaps a little thickened or widened, but that is all (it will not be so if it is a good long or well dispersed one, such as is produced by employing two bisulphide prisms). Each colour, proceeding from the red end, simply appears more and more bent up. Hence the "dispersion," or opening out of the beam of white light into a spectrum of various colours, may be accounted for on the hypothesis of a different refrangibility for each colour, supposing only we find on experiment that such a combination of colours will compose white light. 9 43. Each Colour has its own Angle of Total Reflection.-Newton proved the special refrangibility of each colour by another still more beautiful experiment; one of the most elegant ever devised. It depends on the facts already noticed, that the angle of total reflection must vary with the index of refraction (§ 32); the violet rays being totally reflected (because more refracted), at an angle which would allow the red rays to leave the denser medium. Newton therefore arranged an experiment as in Fig. 50, except that he employed the parallel rays of the sun instead of those from the lime-light lantern. A perpendicular slit N is placed in the optical stage with the objective removed, or on the nozzle of the lantern if an adjustable slit is at command, (see Fig. 50, which shows all the arrangements in plan). As close to the slit as convenient, on a table-stand, simply "stood up" on their ends, are two similar right-angled glass reflecting prisms, P and P2, with their reflecting sides together, kept together by an elastic band passed round near each end; they must not, however, quite touch, and may, if necessary, be kept apart by a narrow slip of paper at each end between them. In the direct path of the rays from the slit, is a focusing lens, F, and beyond that, on another |