As photographers, the matter of colour exercises us but indirectly, still the subject should be understood, on account of its bearing on painting. "Colour perception," says Le Conte, "is a single perception, and irresolvable with any other. It must, therefore, have its basis in retinal structure."

Helmholtz divides the vibrations of ether known as light into three degrees. He says the longest and shortest rays do not essentially differ in any other physical property, except that we distinguish them from the intermediate waves." Thus the ear can receive at once many waves of sound or notes, and they remain distinct, but notes of colour do not keep distinct in the same way, "so that the eye is capable of recognizing few differences in quality of light," says Helmholtz, and can only perceive the elementary sensation of colour by artificial preparation. He also says, the only bond between the objective and subjective phenomena of colour may be stated as a law thus, "Similar light produces under like conditions a like sensation of colour. Light, which under like conditions, excites unlike sensations of colour is dissimilar;"what we want in art, then, is the appearance of the phenomena. The illumination of the sun's rays cannot be weakened without at the same time weakening their heating and chemical action; this is a point to be remembered in exposure.

Colour is, of course, excited by the length of the waves and their frequency, red being the longest and slowest, and they diminish in length and increase in frequency in the order of the spectrum through orange, yellow, green, blue, indigo, to the shortest waves, which produce the effect of violet, the whole combined forming white. Now Hering has shown that there are only four primary colour sensations, though he at one time included black and white, thus making six. The four are red, yellow, green, and blue, which are reduced by him to two complementary colours, red and green, and yellow and blue. In our present state of knowledge the Young-Helmholtz theory of three primary colour sensations for red, green, and blue seems preferable as a working hypothesis, though it seems incompatible with anatomical and physiological facts.

Difference Of Colour

All objective differences between colours, according to Helmholtz, may be reduced to differences of tone, difference of fulness (saturation), and difference of brightness. These are the three colour constants.

By tone, or hue, he means in fact difference of colour as in the spectral colours. He here refers to the vibration on a tonic scale. Fulness or purity is greatest in the pure tints of the spectrum, and becomes less in proportion as they are mixed with white light. All compound colours are less full than the simple hues of the spectrum.

Brightness or luminosity is strength of light, or amount of illumination. It is measured by the total amount of light reflected to the eye.

In nature black and white must be included among the primary colours when quality is spoken of, as light acts on black and white.

All differences of tone, therefore, are the result of combinations in different proportions of the four primary colours.

Among the defects of the eye in seeing colour, Helmholtz says, "All are red blind at the innermost portion of the Held of vision, all red colours appear darker when viewed indirectly."

The furthest limit of visible field is a narrow zone, in which all distribution of colour ceases, and there only remain differences of brightness. Probably those nervous fibres which convey impressions of green light are alone present in this part of the retina. The yellow spot makes all blue light appear somewhat darker in the centre of the field.

All these inequalities are known and more or less rectified by constant movement. As the eye becomes fatigued by bright light, so that it cannot at first answer to delicate stimulus, so it can become partially fatigued for certain colours.

Fatigue weakens the apparent illumination of the entire field of vision.

The colour of illumination of a picture, too, varies greatly by effect of local colour.

What is constant in the colour of an object is not the brightness and colour of the light which it reflects, but the relation between the intensity of the different-coloured constituents of this light, on the one hand, and that of the corresponding constituents of the light which illuminates it on the other. For example, white paper in full moonlight is darker than black satin in daylight, or a dark object with the sun shining on it reflects light of exactly the same colour, and perhaps the same brightness, as a white object in shadow. Grey in shadow looks like white.

Brightness of local colour diminishes with the illumination or as the fatigue of the retina is increased. In sunshine, local colours of moderate brightness approach the brightest, whereas in moonlight they approach the darkest. Pictures to be seen in daylight do not admit of difference of brightness between sun and moon. As colours increase in brightness, red and yellow become apparently stronger than blue. Painters make yellow tints predominate when representing landscape in full sunshine, while moonlight scenes are blued. Helmholtz says:- "Differences of colour which are actually before our eyes are more easily apprehended than those which we only keep in memory, and contrast between objects which are close to one another in the field of vision are more easily recognized than when they are at a distance. All this contributes to the effect. Indeed, there are a number of subordinate circumstances affecting the result which it would be very interesting to follow out in detail, for they throw great light upon the way in which we judge of local colour; but we must not pursue the inquiry further here. I will only remark that all these effects of contrast are not less interesting for the scientific painter than for the physiologist, since he must often exaggerate the natural phenomenon of contrast in order to produce the impression of greater varieties of light and greater fulness of colour than can be actually produced by artificial pigments"

Again, when turbidity is composed of fine particles its appearance is blue, as the mists seen in autumn hanging round coverts, but it is whiter than the aerial blue because of the colour of the covert behind. When this turbidity is absent the colours are brighter, hence the fierce blue on bright sunshiny days with easterly winds. This matter of turbidity must not be forgotten in portrait work; it is this which helps to give relief, hence the absurdity of all photographers' devices, the object of which is to minimize this turbidity. In addition to these is the ever-changing effect of atmosphere on colour, that subtle medium with which the enchantress Nature produces ever-changing effects, and its chief effect on colour is to lower it in brightness. Atmosphere greys all things, hence on a misty day all the colours are greyed - we have, in fact, a "grey day."

Another point which must not be forgotten is that with bright illumination bright objects become more like the brightest, and with feeble illumination dark objects become more like the darkest. This is a very important matter, for it means that in bright sunshine the lightest greys are lost in white, whilst in dull weather the darkest greys are lost in black, hence the falsity of having deep blacks in brightly-lighted landscapes, and as has been shown, these are untrue, and the result of ignorance and of faulty manipulation. As Helmholtz has it, "The difference of brightness and not absolute brightness; and that the differences in them in this latter respect can be shown without perceptible incongruity if only their graduations are imitated with expression."