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The result of this contrast is, that the stripes, seen from a suitable distance, resemble channeled grooves (glyphs) more than plane surfaces. For in the stripes 2 and 3, for instance, the grey being insensibly shaded from the edge a a to the edge bb, they present to the eye the same effect as if the light fell upon a channeled surface, so as to light the part near to b b, while the part a a will appear to be in the shade; but with this difference, that in a real channel the lighted part would throw a reflection on the dark part.

(12.) Contrast of Tone occurs with Colours, properly so called, as well as with Grey; thus, to repeat our experiment (9. fig. 1.) with the two portions o', o of a sheet of paper of a light tone of a certain colour, and the two portions p', p of a sheet of paper of a deeper tone of this same colour, we shall perceive that o contiguous to p will be lighter than o', and p will be deeper than p'. We can demonstrate, as we have done before (10.), that in starting from the point of contact the modification is gradually weakened.

Experimental Demonstration of Contrast of Colour.

(13.) If we arrange as before (9. fig. 1.) two portions o, o' of a sheet of unglazed coloured paper, and two portions, p, p of a sheet of unglazed paper of a different colour from the first, but resembling it as nearly as possible in intensity, or rather in tone (8.), in looking at these four half-sheets o', o, P,p' for a few seconds, we shall see that o differs from o', and p from p'; consequently the two half-sheets o, p appear to undergo reciprocally a modification of tint which is rendered apparent by the comparison we have made of their colours with those of o' and of p.* *

(14.) It is easy to demonstrate that the modification which colours undergo by juxtaposition, is a tendency to weakening starting from the line of juxtaposition; and that it may be perceived between two surfaces without their being in contact, it is sufficient to experiment as above (10.).

(15.) I will now cite seventeen observations made in conformity with the method prescribed (9.).

*Instead of paper we may use lustreless stuffs, or any other material which will present two equal surfaces absolutely identical in colour. Coloured paper answers very well.

(The colours experimented with must be as nearly as possible of equal

Colours experimented with. Red

No. 1. {Orange

intensity.)

Modifications.

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It follows, then, from the experiments described in this Chapter, that two coloured surfaces in juxtaposition will exhibit two modifications to the eye viewing them simultaneously, the one relative to the height of tone of their respective colours, and the other relative to the physical composition of these same colours.

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CHAPTER II.

THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS, AND THE FORMULA WHICH REPRESENTS IT.

(16.) AFTER satisfying myself that the preceding phenomena constantly recurred when my sight was not fatigued, and that many persons accustomed to judge of colours saw them as I did, I endeavoured to reduce them to some general expression that would suffice to enable us to predict the effect that would be produced upon the organ of sight by the juxtaposition of two given colours. All the phenomena I have observed seem to me to depend upon a very simple law, which, taken in its most general signification, may be expressed in these terms:

In the case where the eye sees at the same time two contiguous colours, they will appear as dissimilar as possible, both in their optical composition and in the height of their

tone.

We have then, at the same time, simultaneous contrast of colour properly so called, and contrast of tone.

(17.) For two contiguous colours, o and p, will differ as much as possible from each other when the complementary of o is added to p, and the complementary of p is added to o: consequently, by the juxtaposition of o and p, the rays of the colour p, which o reflects when it is seen separately, like the rays of the colour o which p reflects when viewed separately, rays which are active under these circumstances (7.) cease to be so when o and p are placed in contact; for in this case, each of the two colours losing what is analogous, must differ

the more.

The following formula will make this perfectly intelligible. (18.) Let us represent:

The colour of the stripe o by the colour a whiter by B;
The colour of the stripe P by the colour a' whiter by B';
The complementary colour of a by c;

The complementary colour of a' by c'.

The colours of the two stripes viewed separately are

The colour of o = a + B,

The colour of P = a + B'.

By juxtaposition, they become

The colour of o=a+B+c,

The colour of P = a + B' + c.

Let us now show that what this expression amounts to is, that the taking away from the colour a of o the rays of the colour a', and taking away from the colour a' of P the the colour a, we must suppose

white = b,

rays of

B reduced to two portions+white = (a+c).

white = b',

B' reduced to two portions+white = (a + c).

The colours of the two stripes viewed separately are
The colour of o=a+b+ a + c',

The colour of P = a+b+a+c.

By juxtaposition, they become

The colour of o= a+b+c',
a + b' + c,

The colour of =

-an expression which is evidently the same as the first, excepting the values of B and B'.

(19.) I have stated that simultaneous contrast may influence both the optical composition of the colours, and the height of their tone; consequently, when the colours are not of the same degree of intensity, that which is deep appears deeper, and that which is light, lighter; that is to say, the first appears to lose as much of white light as the second seems to reflect more of it.

Therefore, in looking at two contiguous colours, we have Simultaneous Contrast of Colour, and

Simultaneous Contrast of Tone.

CHAPTER III.

THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS, DEMONSTRATED BY THE JUXTAPOSITION OF A GIVEN NUMBER OF COLOURED SUBSTANCES.

(20.) LET us now apply the formula to the seventeen observations of Chap. I., and we shall see that the modifications of contiguous colours are precisely such as would result from the addition to each of them of the colour which is complementary to its neighbour (18.), remarking, that I shall not review these observations in the order they were arranged above: the reader will learn the reason in Chap. IX. But it will always be easy to find the position they occupy in Chap. I., because I have appended to each its consecutive number. For the complementaries of these colours, I refer to the Introduction to Part I. (6.)

Orange and Green. No. 7.

(21.) Blue, the complementary of Orange, added to Green, makes it bluer, or less yellow.

The complementary of Green (Red), added to Orange, the latter becomes redder, or less yellow, and brighter.

Orange and Indigo. No. 8.

(22.) Blue, the complementary of Orange, added to Indigo, makes it bluer, or less red.

The complementary of Indigo (Orange-Yellow), added to Orange, renders it yellower, or less red.

Orange and Violet. No. 9.

(23.) Blue, the complementary of Orange, added to Violet, makes it incline to indigo.

The complementary of Violet (Greenish-Yellow), added to Orange, the latter becomes yellower.

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