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was obtained by using a superb plate for which he was indebted to Mr. Rutherfurd. The plate contained nearly 19,000 lines to the English inch, and was silvered on the back, so that the colours were as bright as those from a glass prism. The spectrum selected for use was nearly six times as long as that furnished by the glass prism-a circumstance, of course, that favoured accurate observation.

The tables that have just been given enable us very easily to calculate the lengths of the waves of light, corresponding to the centres of the coloured spaces in the normal spectrum. It is only necessary to ascertain the number corresponding, for example, to the centre of the red space, then to multiply it by 3.653, and to subtract the product from 7,603 : the result will be the wave-length corresponding to that part of the normal spectrum, expressed in tenmillionths of a millimetre. The following table contains the wave-lengths corresponding to the centres of the col

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The results here given differ somewhat from those obtained by Listing in 1867 ; the differences are partly due to the terms employed ; the author, for example, dividing up into orange-red, orange, and orange-yellow, a space which is called by Listing simply orange. According to the author cyan-blue falls on the red side of the line F; it is placed by Listing, however, on the violet side of this line. Other less important differences might be mentioned; but, as a discussion of them would be out of place in a work like the present, the curious reader is referred for further information to Listing's paper.*

A little study of the normal spectrum, Fig. 4, will enable us to answer some interesting questions. We have already seen that change in colour is always accompanied by change in the length of the waves of light producing it ; hence if we begin at one end of our normal spectrum where the colour is red, and the length of the waves equal to 7,603 tenmillionths of a millimetre, as we diminish this length, we expect to see a corresponding change in the colour of the light : small changes we anticipate will produce small effects on the colour, large changes greater effects.

Now, the question arises whether equal changes of wavelength actually are accompanied by equal alterations of hue in all parts of the spectrum. To take an example : in passing from the orange-yellow, through the pure yellow and greenish-yellow well into the yellow-green region, we find it necessary to shorten our wave-length about 400 of our units ; now will an equal curtailment in other regions of the spectrum carry us through as many changes of hue? The answer to this is not exactly what we might expect. In a great part of the red region a change of this kind produces only slight effects, the red inclining a little more or less to orange, and the same is true of the blue and violet spaces, the hue leaning only a little toward the blue or violet side, as the case may be. Hence it seems that the eye is far more sensitive to changes of wave-length in the middle regions of the spectrum than at either extremity. This circumstance, to say the least, is curious ; but, what is more to our purpose, it is a powerful argument against any theory

* Poggendorff's “ Annalen,” cxxxi., p. 564.

of colour which is founded on supposed analogies with music. But more of this hereafter.

In the prismatic spectrum and in our normal spectrum we found no representative of purple, or purplish tints. This sensation can not be produced by one set of waves alone, whatever their length may be ; it needs the joint action of the red and violet waves, or the red and blue. All other possible tints and hues find their type in some portion of the spectrum, and, as will be shown in the next chapter, this applies just as well to the whole range of browns and greys, as to colours like vermilion and ultramarine.

We have seen that the mixture of long and short waves which compose white light can be analyzed by a prism into its original constituents : the long waves produce on us the

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sensation that we call red, and, as we allow shorter and shorter waves to act on the eye, we experience the sensations known as orange, yellow, green, blue, and violet. When, on the other hand, we combine or mix together these different kinds of light, we reproduce white light. There are a great many different ways of effecting this recomposition ; one of the most beautiful was contrived several years ago by Professor Eli Blake. The spectrum is received on a strip of ordinary looking-glass, which is gently bent by the hands of the experimenter till it becomes somewhat curved ; it then acts like a concave mirror, and can be made to concentrate all the coloured rays on a distant sheet of paper, as shown in Fig. 5. The spot where all the coloured rays are united or mixed appears pure white.

CHAPTER III.

THE CONSTANTS OF COLOUR.

The tints produced by Nature and art are so manifold, often so vague and indefinite, so affected by their environment, or by the illumination under which they are seen, that at first it might well appear as though nothing about them were constant ; as though they had no fixed properties which could be used in reducing them to order, and in arranging in a simple but vast series the immense multitude of which they consist.

Let us examine the matter more closely. We have seen that when a single set of waves acts on the eye a coloursensation is produced, which is perfectly well defined, and which can be indicated with precision by referring it to some portion of the spectrum. We have also found that when waves of light, having all possible lengths, act on the eye simultaneously, the sensation of white is produced. Let us suppose that by the first method a definite coloursensation is generated, and afterward, by the second method, the sensation of white is added to it: white light is added to or mixed with coloured light. This mixture may be accomplished by throwing the solar spectrum on a large sheet of white paper, and then casting on the same sheet of paper the white light which is reflected from a silvered mirror, or from an unsilvered plate of glass. Fig. 6 shows the arrangement. By moving the mirror M, Fig. 6, the white band of light may be made to travel slowly over the whole spectrum, and thus furnish a series of mixtures of

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