a considerable reversed spectrum. Hence we have the power of constructing "direct" prisms which give a spectrum without refracting the beam of light. Direct vision prisms composed of from three to five prisms of glass, are largely used in direct vision spectroscopes. For lantern work such prisms are very expensive, a large size being required; but Mr. C. D. Ahrens has introduced a prism as in Fig. 61, which answers our purposes at a very moderate price. GG are prisms of light glass, enclosing between them bi-sulphide of carbon, B. One made for me gives a dispersion more than equal to a prism-bottle of 60° without any deflection at all, and is not only very handy to work with, as obviating any turning of the lantern aside, but more light

passes through, and the spectrum is not at all curved.1 Just as these pages were written, Mr. Ahrens brought me for trial another bi-sulphide prism made as in Fig. 62. Here G is one equilateral prism of glass, projecting into the cell of fluid B B. There being only one ordinary glass prism here, this is the cheapest large compound prism that can be made. It gives enormous dispersion-about 60 per cent. more than that of a prism-bottle-with very little, if any, more than

1 With a single prism there is a perceptible curvature in the transverse lines or edges of a spectrum projected on a screen, owing to the convergence, and consequent various angles, of the rays traversing it. To get rid of this curvature in spectroscopes is the object of the collimating lens. This brings to parallelism the diverging rays from the slit adjusted at its focus, which then traverse the prism at the same angle.

ordinary deflection, and it may probably be supplied unmounted for about 40s. Much more light traverses this prism than can get through the two prism-bottles generally used when great dispersion is required.

55. Anomalous Dispersion.-We have not even yet got to the bottom of this matter, however. We may obviously construct prisms of different substances, such as water or crown glass, and flint glass, of such angles that their respective spectra shall be of equal length. But if we do so we find the two spectra do not agree. There is more dispersion in one region than in another, as produced by one substance compared with the other; and hence perfect achromatism is impossible with only two prisms or two lenses.

But stranger phenomena still await us. There is a purplish-red aniline dye called Fuchsine. Obtain a little of it, and fill a prism-bottle with a dilute solution of the Fuchsine in alcohol. A bottle of the ordinary 60° angle can be made to answer, but if one of 25° or 30° can be procured it is rather better. There must also be provided a glass trough with parallel sides sufficiently wide apart to contain the prism-bottle; and this also, after the bottle is inserted, must be filled with the same alcohol that dissolves the Fuchsine. It will easily be understood that when the prism-bottle is immersed, both the refractive and dispersive powers of the alcohol in the bottle are exactly neutralized by that in the trough. This peculiar adjustment is necessary to obtain the Fuchsine action separately; for if we use the bottle of Fuchsine alone, supposing we fill it with a strong solution, the absorption is so strong that only the red rays get through (see § 69), while, if we dilute it, as we must do to get more of the spectral colours, any abnormal dispersion. of the Fuchsine is overpowered by the ordinary dispersion of the alcohol. By exactly neutralizing the latter, then, we can employ a dilute solution, and still trace the effect by the

1

lime light. Success will now depend upon attaining by trial a proper strength, and passing the rays from the slit through the Fuchsine as close as possible to the refracting angle, or thinnest portion of coloured fluid. But when these matters are properly adjusted, it will be seen that the order of the spectrum is totally altered, we may almost say reversed. Counting from the red end, instead of beginning with red, the Fuchsine spectrum begins with blue; goes on to violet; then (after the absorption of some colours altogether, a matter we must study later on) comes red in the middle, ending with yellow; green being absent. How many of the colours can be discerned depends on the strength and thickness of solution.

Many other substances give similar phenomena, one of the best being cyan blue. Though startling, however, these appearances are not really more wonderful, when we attentively ponder them, than that dispersive power should, compared with refractive power, differ at all in various substances. Our experiment with the Fuchsine simply shows us in a more exaggerated and startling form, the very same fact-whatever it is—which makes the dispersion of flint differ from that of crown glass. All alike reveal the "anomalous dispersion" of light. We can no longer maintain that the colours have even an invariable order of refrangibility. This is generally the case; but we have now found that sometimes they have not.

And at this stage we must pause a moment to collect our

1 For private observation only, two small slips of glass may be inclined at an angle of say 10° by a strip of wood placed between them at one edge, and a drop of strong Fuchsine solution placed between them. Through this prism a brilliantly-lighted slit may be observed from a good distance; and through such a small thickness even a solution strong enough to overpower the dispersion of the alcohol will allow sufficient of the colours to pass.

ideas. We accounted for reflection by a rough working hypothesis, which for years was more or less accepted, and was known as the Emission or Corpuscular Theory of Light. But even then we found some difficulties in it. These are now vastly increased in many ways; and we find ourselves once more, by the mental constitution bestowed upon us, bound to ask the question: What is Light? We must frame some intelligible hypothesis by which we may string together at least the foregoing facts, and which, if possible, may also account for such phenomena as we may yet further discover. This, then, will be the subject of the following chapter.

CHAPTER V.

WHAT IS LIGHT?--VELOCITY OF LIGHT.-THE UNDULATORY

THEORY.

Light has a Velocity-Velocity implies Motion of some sort-The Emission Theory-Transmission or Motion of a State of Things— Transmission of Wave Motion-Illustrations-Wave-motion and Vision-Analysis of Wave Propagation-The Ether-Refraction according to the Wave Theory-Total Reflection, Dispersion, and Anomalous Dispersion-Mechanical Illustrations.

56. Light has a Velocity. If a man strikes a bright light say ten miles off, we see it so instantaneously that it is very difficult at first not to believe that we are indeed, in some mysterious way, conscious of it the very instant it happensthat time has nothing to do with the matter. And that was probably the most ancient idea of the manner in which we "see" things.

But this idea was necessarily abandoned for ever after a discovery made by Roemer in 1676. He found that, taking the calculated time for the eclipses of Jupiter's satellites, they always took place eight minutes earlier when the earth was nearest to them, and eight minutes later when it was furthest away; whilst if the earth was at either of the midpoints, they happened at the average time. He very soon drew the unavoidable conclusion, that light required about