physical and chemical action is still present beyond these visible boundaries. The invisible, or ultra-red portion, gives evidence of considerable heat, which in this part of the spectrum reaches its maximum, and gradually diminishes in the visible part. The violet end has the power of exerting strong chemical action upon certain compounds, such as iodide and chloride of silver; therefore a photographic spectrum extends considerably beyond the violet. It has, however, been shown by Helmholtz, that, under certain conditions, we can also perceive the ultra-violet end of the spectrum with the eye. If we separate the ultra-violet part of a spectrum, thrown on a screen in a darkened room, by allowing it to pass through a slit in the screen, and if it is then allowed to pass a second time through a prism, which purifies it from all strange rays by refraction, it produces upon the eye the sensation of a glimmer of light, which has a lavender grey colour.

The eye, as is well known, is only to a certain extent sensitive to light-vibrations. Physicists conclude that all the rays in the spectrum, visible as well as invisible, which are situated beyond the red and violet, consist of vibrations of the ether, which differ only in their wave-lengths, and are otherwise quite similar in form; of their vibrations, however, only those of intermediate length act upon the retina, while it is insensible to the vibrations of the ether of greater or less wave-lengths. What we, therefore, distinguish as light and colour arises from a subjective property of the retina, inasmuch as it only reacts on certain ether-vibrations. We might, therefore, imagine the existence of eyes, which could not perceive the inter

mediate parts of the spectrum, as ours can, but only the rays situated at the invisible ends. To such eyes the world would have quite a different aspect.

All the colours which are found in nature or are prepared artificially, simple or compound, can be composed from the colours of the spectrum. Further, white is formed by the combination of all the colours of the spectrum, in the proportion in which they occur in the spectrum. The constituents, therefore, of all the light which the eye can see are contained in the spectrum.

In order to discover what impression the mixture of colours would have on the eye, it was formerly thought sufficient to mix together colouring matters, such as are used in painting, or coloured liquids. It was, however, an error to suppose that by this means the true mixture of colour was obtained; that is to say, the same colour which our eye would perceive, if the two simple colours acted upon it simultaneously. If, for instance, we mix together a blue and yellow powder, we obtain green. We should, however, never produce a green if we mixed together blue and yellow light-rays. The cause of this difference is the following. A colour, such as is used in painting, consists of minute particles, which are transparent, inasmuch as they transmit a certain coloured kind of light, and absorb the remainder. If we now consider light falling upon a greater number of particles, then a small part of it, consisting of white light, is reflected from the surface; the greater part, however, penetrates the outer layer, and is reflected from the surface of particles which are situated at a greater depth. This light is coloured, i.e. the residue of the white light is absorbed. A yellow powder, therefore, absorbs all the rays with

the exception of the yellow; yet this is not absolutely the case, for rays which are situated near the yellow, among others some green, are transmitted also.

The case is the same with the blue powder. It also transmits some green, as well as blue, rays, and absorbs the rest. Now mix the small blue and yellow particles together. The blue rays will be absorbed by the yellow, and the yellow rays by the blue colouring matter, so that the blue and yellow almost disappear in the mixture of the colours, and only the green remains, which is reflected, since it is transmitted to a considerable extent by both bodies. The action would be exactly the same if we looked through a blue and a yellow glass placed side by side. The colour seen is green, for the blue glass only transmits blue and some green, but no yellow, while the yellow absorbs the blue completely, and only transmits green.

The phenomena, are, however, entirely different, if we allow two colours to act on the eye simultaneously, which may be done in the following manner. A spectrum is thrown upon a screen in which two slits are made; through these two slits two rays of simple colours pass, and can be united by a lens. This is the most perfect method of combining colours.

An easier method of combining colours is the Colour Top.

This instrument consists of a disc, which can be made to rotate round a vertical axis, as represented in fig. 29. One hand holds a handle, which supports the upper end of the axis, and the other forcibly draws off a string which is wound round the axis, thus setting the top in rapid rotation. On the top a paper disc has been

fixed, sections of which, as may be seen from the figure, have been painted with the colours which are to be combined. The images of the colours are so rapidly super

Fig 29

have black and white sections in the disc, the result is a grey colour; and by making use of other colours, we can produce mixtures of every possible shade, which can be made darker by the addition of black.

The following is a very simple method of mixing two colours together, which can be done without any instrument. Two coloured wafers, b and c (fig. 30), are placed on the table a certain distance apart, and a small plate of glass is placed before the eye, so that the wafer b is seen directly through it, and at the same time a reflected

Fig. 30.

image of the wafer c, so that both images are superposed, and their colours combined. In this case also we produce a true mixture of colours.

Now all these methods give concordant results, although the second method rests on a different principle to the two others. In the latter the different colours were mixed objectively before they entered the eye; by the coloured top, however, the differently coloured rays were not mixed, but they only fell upon the retina in very rapid succession, the combination of the colours thus taking place on the retina. This, however, shows us