THIRD GROUP.-Two Simple Colours.

1. Red and Yellow.

Red, in losing yellow, appears bluer; and the Yellow, by losing red, appears bluer; or, in other words, the Red inclines to purple, and the Yellow to green.

2. Red and Blue.

Red, in losing blue, will appear yellower; and the Blue, in losing red, will appear yellower; or, in other words, the Red inclines to orange, and the Blue to green.

3. Yellow and Blue.

Yellow, in losing blue, will appear redder; and Blue, in losing yellow, will appear more violet; or, in other words, the Yellow inclines to orange, and the Blue to violet.

FOURTH GROUP.-Two Compounds having the same Simple Colours.

Indigo and Violet.

As Indigo only differs from Violet in containing a larger proportion of blue in comparison to the red, it follows that the difference will be very considerably increased by the Indigo losing red and inclining to a greenish-blue, whilst the Violet, acquiring more red, will incline to this colour. It is evident that if the Violet loses its red, or the Indigo gains it, the two colours will approximate; but as they differ from each other, the first-named effect will ensue.

We may further explain the preceding phenomena by considering Indigo, relatively to the Violet, as blue; then it will lose its blue, that being common to both colours, and incline to green, and the Violet, in also losing some blue, will appear redder.

FIFTH GROUP.-A Compound Colour and a Simple Colour which is not found in the Compound.

1. Orange and Blue.

2. Green and Red.

3. Violet and Greenish-Yellow.

If we adopt the hypothesis that orange, green, indigo, and violet are compound, and red, blue, and yellow simple colours, it necessarily follows, that in opposing them in the order in which they are reciprocally complementary, and in supposing also that the colours thus in juxtaposition are perfectly free from any foreign colour, we cannot see any reason why the compound colour should lose one of its colours rather than the other, or why the simple colour should separate itself from one elementary colour rather than from the other. For example, in the juxtaposition of green and red, we can see no reason why the green should tend to blue rather than to yellow, or why the red should incline to blue rather than to yellow.

SECTION II.

ON THE DISTINCTION BETWEEN SIMULTANEOUS, SUCCESSIVE, AND MIXED CONTRAST OF COLOURS; AND ON THE CONNECTION BETWEEN THE EXPERIMENTS OF THE AUTHOR AND THOSE MADE PREVIOUSLY BY OTHER OBSERVERS.

CHAPTER I.-DISTINCTION BETWEEN SIMULTANEOUS, SUCCESSIVE, AND MIXED CONTRAST OF COLOURS (77).

CHAPTER II.-ON THE CONNECTION BETWEEN AUTHOR'S EXPERIMENTS AND THOSE PREVIOUSLY MADE BY VARIOUS NATURAL PHILOSOPHERS (120).

CHAPTER I.

DISTINCTION BETWEEN SIMULTANEOUS, SUCCESSIVE, AND MIXED CONTRAST OF COLOUurs.

(77.) BEFORE speaking of the connection between my own observations and those of others, upon contrast of colours, it is absolutely necessary to distinguish three kinds of contrast.

The first includes the phenomena which relate to the contrast I.name simultaneous;

The second, those which concern the contrast I call successive;

The third, those which relate to the contrast I name mixed.

(78.) In the simultaneous contrast of colours is included all the phenomena of modification which differently coloured objects appear to undergo in their physical composition and in the height of tone of their respective colours, when seen simultaneously.

(79.) The successive contrast of colours includes all the phenomena which are observed when the eyes, having looked at one or more coloured objects for a certain length of time, perceive, upon turning them away, images of these objects, having the colour complementary to that which belongs to each of them.

(80.) I hope to prove by the details which I shall give in the following chapter, that in default of this distinction, one of the subjects of optics, the most fruitful in its applications, has not generally been treated with that precision and clearness necessary to impress its importance on those who, not making any observations of their own, have been contented with reading the result of my researches up to the year 1828, which I presented to the Academy of Sciences. In fact, the distinction of the three contrasts will render it easy to appreciate what new facts my researches add to the history of

vision, and to the applications deduced from the study of contrasts. I shall also add, that Dr. Plateau, of Belgium, who has occupied himself for many years in reducing all these phenomena to a mathematical and physiological theory, has adopted the distinction of simultaneous and successive contrasts in his own writings.

(81.) The distinction of simultaneous and successive contrast renders it easy to comprehend a phenomenon which we may call the mixed contrast; because it results from the fact of the eye, having seen for a time a certain colour, acquiring an aptitude to see for another period the complementary of that colour, and also a new colour, presented to it by an exterior object; the sensation then perceived is that which results from this new colour and the complementary of the first.

(82.) The following is a very simple method of observing the mixed contrast.

One eye being closed, the right for instance, let the left eye regard fixedly a piece of paper of the colour a: when this colour appears dimmed, immediately direct the eye upon a sheet of paper coloured B; then we have the impression which results from the mixture of this colour в with the complementary colour (c) of the colour A. To be satisfied of this mixed impression, it is sufficient to close the left eye, and to look at the colour в with the right: not only is the impression that produced by the colour B, but it may appear modified in a direction contrary to the mixed impression c+B, or, what comes to the same thing, it appears to be more A+B.

In closing the right eye and regarding anew the colour B with the left eye, many times running, we perceive different impressions, but successively more and more feeble, until at last the left eye returns to its normal state.

(83.) If instead of regarding в with the left eye, which becomes modified by the colour A, we observe B with both eyes, the right eye being in the normal state, the modification represented by c+B is found much weakened, because it is really c+B+B.

(84.) I advise every one who believes he has one eye more sensitive to the perception of colours than the other,

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