out into a fan shape, as is indicated by the dotted lines, and forms on the screen an oblong image of the most splendid colours. In this beautiful result, two facts, which are wholly distinct, must be remarked:-1st, the light is refracted or bent out of its rectilinear path; 2nd, it is dispersed into an oblong coloured figure.

On examining this figure or image, which passes under the name of the solar spectrum, we find it divided into seven well-marked regions. Its lowest portion, that is to say, the part nearest to that to which the light would have gone had not the prism intervened, is of a red colour, the most distant is of a violet, and between these other colours may be seen occurring in the following order: red, orange, yellow, green, blue, indigo, violet.

In Fig. 241 the order in which they occur is indicated by their initial letters, e being the point to which the light would have gone had not the

prism intervened.

Now, from what source M
do these splendid colours
come ?
Newton proved

that they pre-existed in

Fig. 241. the white light, which,

in reality, is made up of

them all taken in proper proportions.

There are many ways in which this important truth can be established. Thus, if we take a second prism, B B' S', Fig. 242, and put it in an inverted position as respects the first, A A'S, so that it shall refract again in the opposite direction in the rays refracted by the first, they will, after this second refraction, reunite and form a uniform beam, M, of white light, in all respects like the original beam itself.

If the production of colour were due to any irregular action of the faces of the first prism, the introduction of two more faces in the second prism would only tend to increase the coloration. But so far from this, no sooner is this second prism introduced than the rays reunite and recompose white light. It follows as an inevitable consequence that white light contains all the seven

rays.

But Newton was not satisfied with this. He further collected the prismatic coloured rays together into one focus by means of a lens, and found that they produced a spot of dazzling whiteness. And when he took seven powders of colours corresponding with the prismatic rays, and ground them intimately together in a mortar, he found that the resulting powder had a whitish aspect; or if, on the surface of a wheel which could be made to spin round very fast on its axis, coloured spaces were painted, when the wheel was made to turn so that the eye could no longer distinguish the separate tints, the whole assumed a whitish-gray appearance.

By many experiments Newton proved that the true cause of this development of brilliant colours from a ray of white light by the prism is due to the fact that that instrument does not refract all the colours alike. Thus it could be completely shown, in the case of any transparent medium, that the violet ray was far more refrangible than the red, or more disturbed by such a medium from its course. In this originated the doctrine of "the different refrangibility of the rays of light."

On examining the order of colours in the spectrum, we find, in reality, as in Fig. 241, that the red is least disturbed from its course, and the other colours follow in a fixed order. The red, therefore, is spoken of as the least refrangible ray, the violet as the most, and the other colours as intermediately refrangible.

We now see the cause of the development of these colours from white light, which contains them all. If the prism acted on every ray alike, it would merely produce a white spot at d, analogous to that at e, Fig. 241; but as it acts unequally, it separates the coloured rays from one another, and gives rise to the spectrum.

On examining prisms of different transparent media, we find that they act very differently-some dispersing the rays far more powerfully than others, and giving rise, under the same circumstances, to spectra of very different lengths. In the treatises on optics, tables of the dispersive powers of different transparent bodies are given; thus it appears that oil of cassia is more dispersive than rock-salt, rock-salt more than water, and water more than fluor spar.

Moreover, in many instances, it has been found that if we use different prisms which give spectra of equal lengths, the coloured spaces are unequally spread out. This shows that media differ in their refracting action upon particular rays, some acting upon one colour more powerfully than another. This is called irrationality of dispersion.

The different coloured rays of light are not equally luminous— that is to say, do not impress our eyes with an equal brilliancy. If a piece of finely-printed paper be placed in the spectrum, we can read the letters at a much greater distance in the yellow than in the other regions, and from this the light declines on either hand, and gradually fades away in the violet and the red.

It has also been found that the colours are not continuous throughout, but that when delicate means of examination are resorted to, the spectrum is seen to be crossed with many hundreds of dark lines, irregularly scattered through it. A representation Fig. 243. of some of the larger of these is given in Fig. 243. It is curious that though they exist in the sun-light, and in that of the planets, they are not found in the spectra of ordinary artificial lights; and, indeed, the electric spark gives a light which is crossed by brilliant lines instead of black ones. The chief fixed lines are designated by the letters of the alphabet, as shown in the figure.

The light of the sun is accompanied by heat. Dr. Herschel found that the different coloured prismatic spaces possess very different power over the thermometer. The heat is least in the violet, and continually increases as we descend through the colours, the red being the hottest of them all. But below this, and out of the spectrum, when there is no light at all, the maximum of heat is found. The heat of the sunbeam is therefore refrangible, but is less refrangible than the red ray of light.

Late discoveries have shown that every ray of light can produce specific changes in compound bodies. Thus it is the yellow ray which controls the growth of plants, and makes the leaves turn green; the blue ray which brings about a peculiar decomposition of the iodides and chlorides of silver, bodies which are used in photogenic drawing. Those substances which

phosphoresce after exposure to the sun are differently affected by the different rays-the more refrangible producing their glow, and the less extinguishing them.

CHAPTER XL.

OF COLOURED LIGHT.

Properties of Homogeneous Light-Formation of Compound Colours-Chromatic Aberration of Lenses-Achromatic Prism Achromatic Lens-Imperfect Achromaticity from Irrationality of Dispersion-Cause of the *Colours of Opaque Objects-Effects of Monochromatic Lights-Colours of Transparent Media.

EACH colour of the prismatic spectrum consists of homogenous light. It can no longer be dispersed into other colours, or changed by refraction in any manner. Thus, let a ray of

light, S, Fig. 244, enter through
an aperture, F, into a dark room,
and be dispersed by the prism,
A B C; through a hole, G, in a N
screen, D E, let the resulting
spectrum pass, and be received on

a

Fig. 244.

a second screen, de, placed some distance behind; in this let there be a small opening, g, through which one of the coloured rays of the spectrum, formed by A B C, may pass and be received on a second prism, abc; it will undergo refraction, and pass to the position M on the screen, N M. But it will not be dispersed, nor will new colours arise from it; and it is immaterial which particular ray is made to pass the opening at g, the same result is uniformly obtained.

Homogeneous or monochromatic colours, therefore, cannot suffer dispersion.

By the aid of the instrument, Fig. 245, which consists of a series of little

Fig 245.

plane mirrors set upon a frame, we can demonstrate, in a very striking manner, the constitution of different kinds of lights; for if this instrument be placed in such a manner as to receive the prismatic spectrum, by turning its mirrors in a suitable position, we can throw the rays they receive at pleasure on a screen. Thus, if we mix together the red and blue ray, a purple results; if the red and yellow, an orange; and if the yellow and blue, a green. It is obvious, therefore, that of the colours we have enumerated in Chapter XXXIX. as the seven prismatic rays, the green, the indigo, and violet may be compound, or secondary ones, arising from the intermixture of red, yellow, and blue, which by many philosophers are looked upon as the three primitive colours.

We have already remarked that there is an analogy between prisms and

lenses in their action on the rays of light, and have shown how rays become converging or diverging in their passage through those transparent solids. In the same manner it also follows, that as prisms produce dispersion as well as refraction, so, too, must lenses; for, by considering the action of pairs of prisms, as in Fig. 246, or as we have already done in Chapter XXXVIII., we arrive at the action of concave and convex lenses, and find that as refrangibility differs for different rays-being least for the red, and most for the violet a lens acting unequally will cause objects to be seen through it fringed with prismatic colours. This phenomenon passes under the title of chromatic aberration of lenses.

Fig. 246.

To understand more clearly the nature of this, let parallel rays of red light fall upon a plano-convex lens, A B, Fig. 247, and be converged by it to a focus in the point r, the distance of which from the lens is measured. Then let parallel rays of violet light, in like manner, fall on the lens,

and be converged by it to a focus,

v.

On being measured it will be found that this focus is much nearer the lens than the other; and the cause of it is plainly due to the unequal refrangibility of the two kinds of light. The violet

、

B

Fig. 247.

is the more refrangible, and is, therefore, more powerfully acted on by the lens, and made to converge more rapidly.

But this which we have been tracing in the case of homogeneous rays must of course take place in the compound white light. On the same principle that the prism separates the white light into its constituent rays by acting unequally on them, so, too, will the lens. Parallel rays of white light falling on a lens, such as Fig. 247, are not, therefore, converged to one common focus, as represented in Chapter XXXVIII., but in reality give rise to a series of foci of different colours, the red being the most remote from the lens, and the violet nearest.

In some of the most important optical instruments it is absolutely necessary that this defect should be avoided, and that a method should be hit upon by which light may be refracted without being dispersed. Newton, who believed that it was impossible to succeed with this, gave up the improvement of the refracting telescope, in which it is required that images should be formed without chromatic dispersion, as hopeless. But, subsequently, it was shown that refraction without dispersion can be effected. This is done by employing two bodies having equal refractive, but unequal dispersive powers. Those which are commonly selected are crown and flint glass, which refract nearly equally; the index for crown being about 1.53, and that of flint 1.60; but the dispersion of good flint glass is twice that of

crown.

If, now, we take two prisms, A B C, Fig. 248, being of crown, and A CD of flint glass, and place them with their bases in opposite ways, the refracting angle, C, of the latter being half that of A, the former, or, in other words, adjusted to their relative dispersive powers, it will be found

that a ray of light passes through the compound prism, undergoing refraction, and emerging without dispersion; for the incident ray, in its passage through the crown prism, will be dispersed into the coloured rays, and

Fig. 248.

these, falling on the flint prism-the dispersive power of which we assume to be double, and acting in the opposite direction-will be refracted in the opposite direction, and emerge undispersed. Such an instrument is called an achromatic prism.

The same principle can, of course, be used in the construction of lenses, between which and prisms there is that general analogy heretofore spoken of. The achromatic lens consists of a concave lens of flint and a convex one of crown, the curvatures of each being adjusted on the same principle as the angles of the achromatic prism are determined. Such an arrangement is represented in Fig. 249. It gives in its focus the images of objects in their natural colours, and nearly devoid of fringes. But, in practice, it has been found impossible, by any such arrangement, to effect the total destruction of colour. The edges of luminous bodies seen through such lenses are fringed with colour to a slight extent. This arises from the circumstance that the dispersive powers of the media employed are not the same for every coloured ray. The simple achromatic lens, Fig. 249, will collect the extreme rays together, but leaves the intermediate ones, to a small extent, outstanding.

The theory of the compound constitution of light enables us to account, in a clear manner, for the colours of natural objects. Fig. 249. Those which exhibit themselves to us as white merely reflect back to the eye the white light which falls on them, and the black ones absorb all the incident rays. The general reason of coloration is, therefore, the absorption of one or other tint, and the reflection of the rest of the spectral colours. Thus an object looks blue because it reflects the blue rays more copiously than any others, absorbing the greater part of the rest. And the same

explanation applies to red or yellow, and, indeed, to any compound colours, such as orange, green, &c. That coloured bodies do, in this way, reflect one class of rays more copiously than others, may be proved by placing them in the spectrum. Thus, a red wafer seems of a dusky tint in the blue or violet regions, but of a brilliant red in the red rays.

On the same principles we account for the singular results which arise when monochromatic lights fall on surfaces of any kind. Thus, when spirits of wine is mixed with salt in a plate, and set on fire, the flame is monochromatic yellow-that is, a yellow unaccompanied by any other ray. If the variously coloured objects in a room are illuminated with such a light, they assume an extraordinary appearance: the human countenance, for example, taking on a ghastly and deathlike aspect; the red of the lips and cheeks is no longer red, for no red light falls on it; it therefore assumes a grayish tint.

The colours of transparent bodies, such as stained glass and coloured solutions, arise from the absorption of one class of rays, and the transmission of the rest. Thus there are red glasses and red solutions which permit the red ray alone to traverse them, and totally extinguish every other. But, in