its different colours properly arranged, and having their natural or normal luminosities, and in this rectangle we draw the curve furnished by the red glass (Fig. 17). We find that it is highest in the red space; but even here it reaches only about half way up, showing that the luminosity of the transmitted red light is only half as great as that of the same light in the spectrum; in the orange space it falls rapidly off, the curve sinking with a steep slope; after that it runs out into the green and blue, almost to the violet, in such a way as to indicate that the red glass transmits minute quantities of these different kinds of coloured light. The luminosity, then, of all the transmitted rays, except the red, being quite feeble, the light which comes through appears pure red. Making an examination of an orangeyellow glass in the same way, we obtain the curve shown in Fig. 18 this glass, it appears, transmits most of the red,

FIG. 18.-The shaded portion shows the amount of light transmitted by an orangecoloured glass.

orange, and yellow rays, with much of the green and a little of the blue. Here, of course, the orange and yellow rays after transmission make up an orange-yellow hue, and the green and red rays by their union reproduce the same colour, as we shall see in Chapter X. Hence the final colour is orange-yellow, without the least tint of red or green. Taking next a green glass, we obtain another curve, Fig. 19, showing that much green light is transmitted, but along

FIG. 19.-The shaded portion represents the amount of light transmitted by green glass.

with it some red and some blue. Blue glass shows the cyan-blue weakened, the ultramarine-blue and violet strong; the green is very weak, so also are yellow and orange; the red is mostly absent, except a feeble extreme red. The result is of course a violet-blue (Fig. 20). A purple glass is

found to absorb the middle of the spectrum, i. e., the yellow, green, and cyan-blue; the red and violet are also enfeebled, but are at all events far stronger than the other transmitted rays. We have, then, as a final result, red, ultramarine-blue, and violet, which being mingled make purple. It is evident from these experiments that the colours produced by absorption are not simple, like those furnished by the prism, but are resultant hues, produced by the mixture of many different kinds of coloured light hav

ing varying degrees of brightness. On this account, and by reason of the tendency of many kinds of stained glass to absorb to a considerable extent all kinds of coloured light presented to them, it happens that stained glass furnishes us with coloured light inferior in purity and luminosity to that obtained by the use of a prism. Nevertheless these colours are the purest and most intense that we meet with in daily life, and far surpass in brilliancy and saturation those produced by dyestuffs or pigments.

There is one property which probably all substances possess which produce colour by absorption, upon which a few words must be now bestowed. If we cause white

light to pass through a single plate of yellow glass, the rays which reach the eye will of course be coloured yellow. Add now a second plate of the same glass, and the light which traverses the double plate assumes a somewhat different appearance; it evidently is not so luminous, and its colour is no longer quite the same. Using six or eight plates of the yellow glass, we find that the transmitted light appears orange. If the same experiment be repeated, using a considerable number of plates of the same glass, the colour will change to dark red. From this it appears that the colour of the transmitted beam of light depends somewhat on the thickness of the absorbing medium. This change in the case of some liquids is very considerable: thin layers, for example, of a solution of chloride of chromium transmit green light mainly, and so imitate the action of a plate of green glass; thick layers of the same liquid transmit less light in general, but the dominant colour is red, and objects viewed through them look as they would, seen through a plate of dark-red glass. This curious property is easily explained by an examination of the action of the liquid on the prismatic spectrum. In Fig. 21 the curve represents the relative intensity of the coloured light in different portions of the spectrum. If we cut off successively slices of the rectangle, as is done in Figs. 22

and 23, we obtain the curves corresponding to a greater and greater thickness of liquid, and it is plain that at last we shall have the state of things indicated in Fig. 23; the

FIG. 22.-Chloride of Chromium; Effect produced by a Thick Layer.

curve is about the same as for red glass (Fig. 17), and the final colour is red. This is an extreme case, but in stained glasses, pigments, dyestuffs, etc., there is generally a ten

FIG. 28.-Chloride of Chromium; Effect produced by a very Thick Layer.

dency toward the production of effects of this kind, some of which will hereafter be noticed.

The colours of painted glass are similar to those of stained glass in origin and properties; both are intense, rather free from admixture with white light, and capable

of a high degree of luminosity. In these respects they far surpass the colours of pigments, which compared with them appear feeble and dull, or pale. Owing to this circumstance, chromatic combinations may be successfully worked out in stained glass, which would prove failures if attempted with pigments or dyestuffs. Hence also the wonderfully luminous appearance of paintings on glass viewed in a properly darkened room: they surpass in some respects oil or water-colour paintings to such a degree that the two are not to be mentioned together. There is no doubt but that glass-painting offers advantages for the production of realistic effects of colour and light and shade, such as the very narrow scale of oil and water-colour utterly denies; and yet great artists seem to reject this process, and severely confine themselves to work on canvas or paper, choosing to depend for their effects rather on pure technical skill and artistic feeling.

If we place on a sheet of white paper a fragment of paleblue glass, it will display its colour, though not so brilliantly as when held so that the light of the window streams through it directly. The reason is very evident: the light which penetrates the glass falls on the paper and is reflected by it back through the glass to the eye. The light then traverses the glass twice, but this is not the only cause of its inferior luminosity, for a double plate of the same glass held before the window appears still far brighter than the single glass on the paper. The other reason is that the paper itself reflects only a small amount of the light falling on it. Upon examining the matter more closely we find also that the blue glass reflects from its surface quite a quantity of white light, which, when mingled with the coloured light, renders it somewhat pale. If, now, we grind up into a very fine powder some of the blue glass, we obtain the pigment known as smalt, and, after mixing it with water, we can wash our white paper with a thin layer of it. When it dries the