objects presenting both colours by daylight appear more or less of a green colour by candlelight.

If the two colours are mingled the mixture presents that colour which remains over after the abstraction of all the rays absorbed by the two materials. It is, for example, generally known that a mixture of blue and yellow, as of Prussian blue and gamboge, produces a green. This is by no means in opposition to the fact above stated (§ 57), that the yellow and the blue of the spectrum unite to form white. For in order that our eyes should receive the impression of white it is necessary that blue and yellow rays should enter them simultaneously. A mixture of Prussian blue and gamboge emits neither blue nor yellow, but essentially green rays. The former colouring matter absorbs the red and yellow, the latter the blue and violet rays, and the green rays therefore alone remain in the diffuse light reflected from the mixture.

FLUORESCENCE.

CHAPTER XIII.

PHOSPHORESCENCE.

CHEMICAL ACTION.

78. THE question now arises, what becomes of the rays that have undergone absorption ? Are they in fact, as they appear to be, annihilated? A series of phenomena now to be considered will give us an answer to these questions.

FIG. 122.

If water containing a little Esculin, a substance contained in the bark of the horse chestnut in solution, be placed in a flask, and the rays of the sun or of the electric lamp concentrated by a lens situated at aboutits focal distance from the vessel, be directed upon it, the cone of light thrown by the lens into the interior of the fluid will be seen to shine with

Fluorescence.

a lovely sky-blue tint. The particles of the solution of Esculin in the path of the beam become spontaneously luminous, and emit a soft blue light in all directions. The cone of light appears brightest at the point where it enters into the fluid through the glass, and quickly diminishes in brilliancy as it penetrates more deeply.

There are great numbers of fluid and solid bodies.

which become similarly self-luminous under the influence of light. This peculiarity was first observed in a kind of spar occurring at Alston Moor in England, which, itself of a clear green colour, appears by transmitted solar light of a very beautiful indigo-violet colour. From its occurrence in Calcium fluoride the phenomenon has been named fluorescence.

In order to understand more precisely the circumstances under which fluorescence occurs, the solution of Esculin must again be referred to. The light before it reaches the lens must be allowed to pass through just such another solution of Esculin contained in a glass cell with parallel walls. The cone of light proceeding from the lens, as long as it passes through the air, does not appear to have undergone any material change, it is just as bright and just as white as before. In the interior of the fluid however it no longer presents a blue shimmer but becomes scarcely perceptible.

Thus it is seen that light which has traversed a solution of Esculin is no longer capable of exciting fluorescence in another solution of Esculin. Those rays consequently which possess this property must be arrested by the first solution of Esculin. Similar results are obtained in the case of every other fluorescent substance.

The general proposition can therefore be laid down, that a body capable of exhibiting fluorescence fluoresces by virtue of those rays which it absorbs.

In order to determine what rays in particular cause the fluorescence of Esculin, the spectrum must be projected in the usual way; but instead of its being received upon a paper screen it must be allowed to fall upon the wall of a glass cell containing a solution of

Esculin, that is to say, upon the solution itself, and it must then be observed in what parts of the spectrum the blue shimmer appears. The red and all the other colours consecutively down to indigo appear to be absolutely without effect. The bluish shimmer first commences in the neighbourhood of the line G, and covers not only the violet part of the spectrum, but stretches far beyond the group of lines H to a distance which is about equal to the length of the spectrum visible under ordinary circumstances.

From this the conclusion must be drawn that there are rays which are still more refrangible than the violet, but which in the ordinary mode of projecting the spectrum are invisible; these are termed the ultra-violet rays. They become apparent in the Esculin solution because they are capable of exciting the bluish fluorescent shimmer in it. If sunlight have been used in the above experiments the well-known Fraunhofer's lines appear upon the bluish ground of the fluorescing spectrum, not only from G to H, but the ultra-violet part also appears filled with numerous lines, the most conspicuous of which are indicated by the several letters L to S

FIG. 123.-Solar spectrum with the ultra-violet portion.

(fig. 123). That these lines, like the ordinary Fraunhofer's lines, belong properly to solar light, and do not depend upon any action of the fluorescing substance, is

evident from the circumstance that with the electric light they are no more apparent in the ultra-violet than in the other colours, and further, because the same lines are seen in the solar spectrum, whatever may be the fluorescing substance under examination.

Quartz has the power of transmitting the ultraviolet rays far more completely than glass. If therefore the glass lens and prism hitherto used for projecting the spectrum be replaced by a quartz lens and prism, the ultra-violet part of the spectrum is rendered much brighter and is extended still further than before.

The ultra-violet rays of the spectrum can, moreover, be seen without the intervention of any fluorescing substance through a glass, or still better, through a quartz prism, if the bright part of the spectrum between B and H be carefully shut off. With feeble illumination its colour appears indigo-blue, but with light of greater intensity it is of a bluish-grey tint (lavender). The ultra-violet rays thus ordinarily escape observation, because they produce a much feebler impression on the human eye than the less refrangible rays between B and H.

An explanation is thus afforded why the solution of Esculin, apart from its absorption, is colourless when seen by transmitted light; for since it absorbs only the feebly luminous violet and the entirely imperceptible ultra-violet rays, the mixed light that has passed through it still appears white and is not rendered. materially fainter.

79. If the solar spectrum be thrown in the abovementioned manner upon the fluid, its fluorescing part everywhere exhibits the same bluish shimmer; and spec