Colour Measurement and Mixture Part 9

That is, the luminosity of the chrome yellow is 78; the same as was obtained by direct measurement.

In the same manner the luminosity of any colour can be found. Thus that of a purple, or of green, can be ascertained; of the former by using the green disc with either the red or the blue disc, and the latter by the red and blue disc. From this it is apparent that we can check the luminosities derived from other means by this plan.

A taking experiment can be made with colour discs to imitate all the colours of the spectrum in their proper order, though diluted more or less by white light. This can be done by rotating V, E, and U together; but in order to get additional luminosity in the yellow, we can use chrome yellow as well. If a disc be made as in the figure (Fig. 44), it will on rotating give a fair imitation of the spectrum, if it be viewed through a slit held in front of the disc.

Fig. 44.--Disc arranged to give approximately all the Spectrum Colours.

The mixture of colours by means of rotating sectors is one which the artist cannot use for artistic purposes, and it might seem that for him any deductions made from this method are useless; but it is not so.

Suppose we take black lines ruled closely together on paper, and examine the surface from such a distance that the lines are no longer distinguishable it will appear of a grey; and if we take the amount of black on the paper and amount of white, and prepare two sectors of black and white, whose angles are in these proportions, and rotate them alongside the ruled surface, it will be found that the grey of one matches the grey of the other. If instead of lines of black and white we have them of light yellow and cobalt blue, a grey is also produced when the surface covered by the blue is to that covered by the yellow in correct proportions, and may be matched by rotating sectors containing merely black and white. Now some artists employ stippling, filling up cross-hatching of one colour with dots of a totally different colour, or they place dots side by side. When seen from the distance at which the picture should be viewed, these various colours blend one into another, and form a tint which is the same as that which would be obtained by rotating these colours together in the proportion in which they cover the ground. Artists, however, generally mix their pigments together on the palette, and the resulting mixtures are often totally unlike those which are obtained by rotating the same colours together, a noteworthy example is that of yellow and blue. By rotation, and when in proper proportion, these two give a white, but when mixed on the palette a green results. What causes this difference? Experimental proof is always the most satisfactory proof, so let us have recourse to the spectrum apparatus to obtain an answer. Let a spectrum be thrown on the screen, and in it place a strip of paper painted with the yellow, and then another with the blue. With the first it will be seen that the blue rays are not reflected, but only the green and yellow and red, taking the spectrum as roughly made up of these four colours. With the latter the yellow is not reflected, and but very little red, but the blue and the green are reflected strongly. Now we have already said that the reflection of colour from a surface is indicative of the colours the particles of pigments when taken thin enough to be transparent would transmit; hence we may take it that the yellow pigment transmits the red, yellow, and green, and the blue pigment scarcely anything but blue and green. When we have a mixture of these fine particles of pigment on paper, some will underlie the others. But let us pay attention to what would happen if a yellow particle were at the top, and a blue one beneath it. White light would impinge on the yellow particle, but only red, yellow, and green would pa.s.s out or be reflected from it. This sifted light would next fall on the blue particle and--as we have seen--only blue and green can pa.s.s through or be reflected from it; but as the yellow particle has already deprived the white light of its blue component, the green light alone would pa.s.s to the paper, and be reflected either direct from the surface of the paper, or through the particles themselves to the eye. If the blue particle were on the top, precisely the same effect would be produced; it would only allow blue and green to pa.s.s to the yellow particle, and as the yellow is opaque to the blue, only green light again would pa.s.s. Similarly if side by side the same phenomena would occur, since the light reflected from one on to the other would be deprived of all colour except the green. A very pretty experimental proof of this is to place a yellow solution of dye in front of the slit of the colour apparatus, and having formed the yellow colour patch to place in it a piece of paper covered with a blue pigment: the latter becomes green. By placing a blue solution in front of the slit, and using a piece of yellow pigmented paper, the same result is obtained. The artist therefore in mixing his pigments calls into play the law of absorption, and from his mixtures very naturally a.s.sumes that blue and yellow make green. He makes a neutral tint of blue, red, and yellow, and as the red cuts off the green, this naturally follows from the above. Such experiments as these led him to the conclusion that red, yellow, and blue are the three primary colours, an a.s.sumption which had he used simple spectrum colours instead of compound colours, such as pigments, he would not have ventured to make.

CHAPTER XVI.

Contrast Colours--Measurement of Contrast Colours--Fatigue of the Eye--After-Images.

Fig. 45.--Method of showing Contrast Colours.

There is a phenomenon in colour which must be alluded to, and which possesses more than a pa.s.sing interest to the art world, and that is colour contrast. Perhaps one of the best methods of showing this is by our colour patch apparatus. If we throw the reflected beam and the colour patch on a square as before, and place a rather thinner rod in front, so that the two shadows lie on a background of the combined white light and spectral colours, on pa.s.sing a slit through the spectrum, the shadow which is illuminated by white light will appear anything but white. Thus if we allow yellow spectral light to illuminate one shadow, the other will appear decidedly of a blue hue; if a green ray it will be of a ruddy hue; if a blue ray of a yellow hue; that is, all the contrast hues will appear to the eye to tend towards a complementary tone to the spectral light. The kind of white light illuminating the shadow has a marked effect on the tone, as might be expected. The following table shows the contrast colour of the white illuminated shadow when the white light used was that of a candle.

+---------------+-------------------+---------------+------------------+ | | Contrast | | Contrast | | Spectrum | Colours in | Spectrum | Colours in | | Colour. | Electric light. | Colour. | Gaslight. | +---------------+-------------------+---------------+------------------+ | Cherry red | Green gray | Cherry red | Green gray | | Scarlet | Bluish green gray | Scarlet | Sap green | | Terra-cotta | Blue gray | Light red | Green gray | | Raw sienna | Light blue gray | Olive green | Pink gray | | Olive green | Umber | Apple green | Mauve & black | | Emerald green | Pinkish lavender | Emerald green | Pink terra-cotta | | Gra.s.s green | Light pink | Emerald green | Pink terra-cotta | | Bluish green | Dark pink | Bluish green |Pinker terra-cotta| | Signal green | Salmon | Peac.o.c.k blue | Salmon | | Cyanine blue | Yellow ochre | Prussian blue | Reddish yellow | | Ultramarine | Raw sienna | Ultramarine | Raw sienna | | Violet blue | Brownish yellow | Violet blue | Brownish Orange | | Blue violet | Green yellow brown| Blue violet | Brownish yellow | | Violet | Burnt sienna | Violet | Yellow ochre | +---------------+-------------------+---------------+------------------+

The contrasts here shown are not so visible when the two shadows of the rod occupy the whole of the white square, but are decidedly increased by the shadows occupying only a part of the field, the margins being illuminated with a mixture of the two lights. Not only are there contrasts with coloured light and white, but the relative position of one colour to another may alter the hue of each to the eye. The following experiments indicate what change can be expected in contrasted colours. The double colour apparatus was used as described at page 122, and a slit was placed in four different positions in the spectrum, viz.

in the red, orange, green, and violet, to form patches, and another slit was placed in the same four positions in the other spectrum, and the contrasts noted.

+-----------------+----------------------------------------------+ |Original Colours.| Change due to Contrast. | +--------+--------+----------------------+-----------------------+ | Red | Orange | Red became yellower | Orange became green | | | | | grey | | " | Green | " unaltered, but | Green unaltered, but | | | | brighter | brighter | | " | Blue | " became more | Blue became greener | | | | orange | | | " | Violet | " became orange | Violet, no marked | | | | | change | | Green | Orange | Green became bluer | Orange became yellower| | " | Blue | " became olive | Blue became more | | | | | violet | | " | Violet | " became yellower| Violet became bluer | | Orange | Blue | Orange became redder | Blue became bluer | | " | Violet | " became greener | Violet became bluer | | Violet | Blue | Hardly altered | Hardly altered | +--------+--------+----------------------+-----------------------+

These contrasts were in most cases very marked, as would be seen by causing the same colours to fall on a different part of the screen, outside that on which the comparisons were made.

This phenomenon of contrast is one which is most valuable for artistic purposes, for it gives a power of increasing the value of the colour of pigments which is used by the artist almost intuitively. Thus he can heighten the tone of his orange pigment, with which he makes a sunset sky, by placing in juxtaposition with it some bit of blue coloured s.p.a.ce. The blue becomes bluer, and the orange more orange, by this artifice. All these artifices--or rather we should say intuitive applications of science--are most necessary when the small range of luminosity of colours with which he has to deal is taken into account.

For instance, in a picture of a sun-lighted snow mountain and deep pine forests, the utmost luminosity he can give to the former is that of white paper when seen in the shade, which, in comparison with what he sees, is really a mixture of 90% of black with the light from the snow, so that his range of luminosity is only nine-tenths of that which occurs in nature. It is in adapting this low scale to his picture that true genius of the artist is seen.

It might seem that these contrast colours being only a physiological effect, could not be accurately measured, but such is not the case, if a little artifice be employed. If we use the second colour patch apparatus side by side with the first, we can very readily and with very close approximation determine the contrast colours we see. Suppose by the second apparatus we form a colour patch of say red, and place a thin rod in the beam of this ray and of the reflected beam, and about six inches from it form another patch with the first apparatus, using the three slits to make colour mixtures; by first noting the contrast colour, and then approximating in the second patch to what the eye perceives, we can little by little get a fairly exact match to the contrast colour, and can definitely note it. We now give the results of three measures made for the contrast colours which presented themselves to the eye when they were caused by a red ray near the lithium line, another near the E line in the green, and the third near the G line in the violet.

To make white light and the contrast colours, the slits had to be of the following apertures--

+-----------------+-------+--------+---------+ | Colour. | Red. | Green. | Violet. | +-----------------+-------+--------+---------+ | White light | 157 | 65 | 98 | | Contrast to Red | 135 | 118 | 225 | | " Green | 158 | 51 | 48 | | " Violet | 159 | 72 | 42 | +-----------------+-------+--------+---------+

Thus to form the contrast to red took 135 of red, 118 of green, and 225 of violet. Now from each of these there can be deducted the amount of white light, which will leave only two colours mixed. Calculating this out we find that the contrasts are--

+-----------------+-------+--------+---------+ | Contrast Colour | Red. | Green. | Violet. | | to | | | | +-----------------+-------+--------+---------| |Red | -- | 35 | 167 | |Green | 157 | 32 | -- | |Violet | 194 | 95 | -- | +-----------------+-------+--------+---------+

If the contrasts were exactly complementary colours, the proportions of the two colours left should be the same as those of the same colours as given, which with the original colour make white light. It will be seen that such is not the case. A very simple way of testing this is to form a patch of white light with the three slits in the first apparatus, and then to obtain the contrasts by the other apparatus, with the same colours one after the other that pa.s.s through the three slits. If now we cover up the slit in the first apparatus through which the colour whose contrast in the second apparatus is sought pa.s.ses, we may dilute it with white light as we will, but in no case has the writer found that an exact match to the contrast colour can be obtained in this way. Thus, supposing we wanted to try the experiment with the same red light as that which comes through the red slit, we should use that same light in the second apparatus, and form the contrast colour with the white beam, and then in the first apparatus cover up the red slit, leaving the violet and green to form a patch on the screen. We should then dilute the colour of this patch with white light, and note if it appeared the same as the contrast colour.

Another phenomenon which presents itself is the fatigue of the colour-sensation apparatus of the eye, induced by looking at a bright object. For instance, if we look at a crimson wafer or spot for some time, and then turn the eye so that it rests on a grey surface, an image of the spot will still be seen, but as of a greenish-blue colour. This is due to the fact that the red-seeing apparatus is fatigued and exhausted, whilst the green and violet-seeing machinery has not been largely exercised. Consequently when looking at grey paper the grey of the paper is seen in the retina at all parts as grey, except in the small part of the retina which has got diminished power of perceiving a red sensation; hence a sea-green image will be seen until the fatigue has pa.s.sed away. This colour can be reproduced with very fair accuracy by allowing only one eye to be fatigued, and then using the other to obtain a colour mixture corresponding to it. It will then be found that the colour is the same as the complementary colour, much diluted with white light.

To the same cause may be traced positive and negative after-images, as they are called. If we look at a strongly-illuminated coloured form, such as a church window, and close the eyes, the window will still be seen, at first of its original colour (a positive after-image), and it will then fade and be seen in its complementary colours (a negative after-image). The positive image is due to the persistence of what we may call nerve irritation, whilst the negative image is due to the physiological excitation of all the nerve fibrils, which ordinarily speaking give the sensation of a very dull white light. The previous fatigue of one set of fibrils, however, prevents them being excited to the same degree as the others, hence we get a complementary image. It would be out of place to pursue this subject further, as we have only dealt with the physical measurement of colour-sensations, and these are beyond it.

THE END.