CHAPTER I.

_Manner of observing the Phenomena of the Simultaneous Contrast of Colours._—_Definition of Simultaneous Contrast._

8. If we look at the same time at two stripes of unequal tints of the same colour, or at two stripes of equal tints of different colours, in juxtaposition, that is to say, contiguous by one of their edges, the eye will perceive, if the stripes be not too wide, certain modifications; in the first case affecting the intensity of the two tints, in the second, the optical composition of the two colours so placed. Now as these modifications cause the colours to appear, when looked at together, more different than they really are, I have given to them the name of the _simultaneous contrast of colours_. The modification which affects the intensity of colour, I term _contrast of tone_; and the modification which affects the optical composition of the contiguous colours, I term _contrast of colour_.

The twofold phenomena of contrast of colour and contrast of tone may be readily shown by the following.

_Experimental Demonstration of Contrast of Tone._

Plate A Aʹ.

9. Let the two halves of a sheet of unglazed paper, about twenty inches square, be coloured clear grey, by a mixture of chalk and black; fix them, in any way, upon a piece of unbleached linen, placed across a window, at the distance of twelve inches asunder. Take two halves of another piece of similar paper B Bʹ, but of a darker grey, and coloured with the same substances. Fix A next to B and place Bʹ twelve inches from B. (See Fig. 1.)

[Illustration: PLATE I.]

Upon looking at the four half-sheets for a few seconds, it will be seen that A contiguous to B will be lighter than Aʹ, while B will seem darker than Bʹ.

10. It is easy to demonstrate that the modification is not equally intense over the whole of the surfaces A Aʹ and B Bʹ, but that it becomes gradually feebler from the line of contact. This may be proved by placing a card, so cut, that O and P may each present three grey stripes, as shown in Fig. 2, Plate 1. The stripes 1 1 are more modified than the stripes 2 2, and these are more so than the stripes 3 3. However, in order that this modification may be effected, it is not absolutely necessary that O and P should touch; for if the stripes 1 1 be covered, the stripes 2 2, 3 3 will be modified.

11. The following experiment, which is simply the result of the two preceding (9 and 10), is well suited to demonstrate the extent of contrast of tone. Upon a sheet of cardboard divided into ten stripes, each about a quarter of an inch broad, lay a uniform tint of Indian ink. As soon as it is dry, lay a second tint on all the stripes except the first. As soon as the second is dry, lay a third on all the stripes except the first and second, and so on of all the rest, so as to have ten flat tints, gradually increasing in depth from the first to the last. (See Fig. 3.) If ten strips of paper of the same grey, but each of a different tone, be laid upon a cardboard, in the preceding gradation, it will serve the same purpose.

Upon looking at the cardboard, it will be seen that the strips, instead of presenting flat tints, will each appear of a tone diminishing in intensity from the edge _a a_ to the edge _b b_. In the stripe 1, the contrast is produced simply by the contiguity of the edge _b b_ with the edge _a a_ of the stripe 2; in the stripe 10, it is simply by the contiguity of the edge _a a_ with the edge _b b_ of the stripe 9. But in each of the intermediate stripes 2, 3, 4, 5, 6, 7, 8, 9, the contrast is produced by a double cause; partly by the contiguity of the edge _a a_ with the edge _b b_ of the stripe which precedes it, partly by the contiguity of the edge _b b_ with the edge _a a_ of the darker tint which follows it. The first cause tends to raise the tone of the half of the intermediate stripe; while the second cause tends to lower the tone of the other half of the same stripe.

In consequence of this contrast, the stripes seen from a proper distance, resemble channels rather than flat surfaces. For, in the stripes 2 and 3 for instance, the grey is weakened from the edge _a a_ to the edge _b b_, presenting to the eye the same effect as if the light fell upon a channelled surface; there is however this difference, that in the real channelling the enlightened part would throw a reflection upon the dark portion.

12. Contrast of tone occurs with colours so called as well as with grey; thus to repeat the experiment (9), fig. 1, with the halves _o o_ of a sheet of paper of a light tint of a certain colour, and the two halves _p p_ of a sheet of paper of a darker tint of the same colour, it will be seen that _o_ contiguous to _p_ will be lighter than _oʹ_, and _p_ darker than _pʹ_. In short, it may be demonstrated as has been done (10) that the modification of colours in juxtaposition becomes weaker in proportion to their distance from the line of contact; and in order to observe this effect in bodies which are not contiguous, it is only necessary to experiment as described in (10).

[Illustration: PLATE II.]

The colours experimented upon must be as nearly as possible of equal intensity.

13. _Experimental Demonstration of Contrast of Colour._—If we arrange as before, the two halves of an unglazed coloured sheet of paper, and two halves of another sheet of a different colour, but as nearly as possible of equal intensity, or rather of _tone_ (8), upon looking at the four half-sheets _o oʹ, p pʹ_ for a few seconds, we shall see that _o_ differs from _oʹ_ and _p_ from _pʹ_; consequently the two half-sheets, _o p_, seem to undergo a reciprocal modification of tint, which is rendered apparent by comparing their colours with those of _oʹ_ and _pʹ_.

14. No 1. { Red inclines to Violet. { Orange ” Yellow. 2. { Red ” Violet. { Yellow ” Green. 3. { Red ” Yellow. { Blue ” Green. 4. { Red ” Yellow. { Indigo ” Blue. 5. { Red ” Yellow. { Violet ” Indigo. 6. { Orange ” Red. { Yellow ” Bright Green. 7. { Orange ” Bright Red. { Green ” Blue. 8. { Orange ” Yellow. { Indigo ” Blue. 9. { Orange ” Yellow. { Violet ” Indigo. 10. {Yellow ” Bright Orange. { Green ” Blue. 11. { Yellow ” Orange. { Blue ” Indigo. 12. { Green ” Yellow. { Blue ” Indigo. 13. { Green ” Yellow. { Indigo ” Violet. 14. { Green ” Yellow. { Violet ” Red. 15. { Blue ” Green. { Indigo ” Deep Violet. 16. { Blue ” Green. { Violet ” Red. 17. { Indigo ” Blue. { Violet ” Red.

15. It thus appears from the experiments described in this chapter that two coloured surfaces in juxtaposition, viewed simultaneously, present to the eye two modifications—one relative to the depth of tone of their respective colours, and the other relative to the physical composition of those colours.