Colour Measurement and Mixture

CHAPTER XVI.. Contrast Colours--Measurement of Contrast Colours--Fatigue of the

Contrast Colours--Measurement of Contrast Colours--Fatigue of the Eye--After-Images p. 196

LIST OF ILLUSTRATIONS.

FIG. PAGE

Colour-patch apparatus Frontispiece

1. Spectrum of sunlight 18

2. Carbon poles of an electric light 20

3. Curve for converting prismatic spectrum into wave-lengths 28

4. The thermopile 35

5. Heating effect of different sources of radiation 38

6. Colour-patch apparatus 42

7. Rotating sectors 45

8. Spectrum of Carbon Sodium and Lithium 48

9. Interference bands 60

10. Absorption of rays by the atmosphere 68

11. Luminosity curve of spectrum of the positive pole of the electric light 79

12. Rectangles of white and vermilion 82

13. Arrangement for measuring the luminosities of pigments 83

14. Measurement of the intensity of rays reflected from white and coloured surfaces 88

15. Intensity of rays reflected from vermilion, emerald green, and French ultramarine 92

16. Method of obtaining two patches of identical colour 95

17. Absorption by red, blue, and green glasses 99

18. Light reflected from metallic surfaces 100

19. Intensities of vermilion, carmine, mercuric iodide, and Indian red 101

20. Intensities of gamboge, Indian yellow, cadmium yellow, and yellow ochre 101

21. Intensities of emerald green, chromous oxide, and terre verte 103

22. Intensities of indigo, Antwerp blue, cobalt, and French ultramarine 104

23. Method of obtaining a colour template 104

24. Template of carmine 106

25. Template of luminosity of white light 108

26. Absorption of transmitted and reflected light by Prussian blue and carmine 107

27. Collimator for comparing the intensity of two sources of light 109

28. Spectrum intensities of sunlight, gaslight, and blue sky 109

29. Comparison of sun and sky lights 111

30. Slide with slits to be used in the spectrum 113

31. Screen on which to match gamboge 116

32. Diaphragm in front of prism 128

33. Curve of sensitiveness of silver bromo-iodide 136

34. Curves of colour sensations 139

35. Knig's curves of colour sensations 151

36. Maxwell's colour-box 152

37. Maxwell's curves of colour sensations 154

38. Chromatic circle 168

39. Disc to cause alternate opening and closing of two slits 179

40. Disc painted blue and red 181

41. Electro-motor with discs attached 183

42. Method of cutting disc to allow an overlap of a second disc 184

43. Arrangement to find value of gamboge in terms of emerald green and vermilion 188

44. Disc arranged to give approximately all the spectrum colours 192

45. Method of showing contrast colours 196

COLOUR MEASUREMENT AND MIXTURE.

CHAPTER I.. Sources of Light--Reflected Light--Reflection From Roughened

Sources of Light--Reflected Light--Reflection from Roughened Surfaces--Colour Constants.

There is nothing, perhaps, in our everyday life which appeals more to the mind than colour, yet so accustomed are the generality of mankind to its influence that but few stop to inquire the "why and wherefore" of its existence, or its cause. To those few, however, there is a source of endless and boundless enjoyment in its study; for in the realms of physical and physiological science there is perhaps no other subject in which experiments give results so fascinating and often so beautiful. Although its serious study must be undertaken with a clear mind, a good eye, and a fair supply of patience, yet a general idea of the subject may be grasped by those who are possessed of but ordinary intelligence.

Colour phenomena are encountered nearly every day of one's life, and the fact that they are so frequently met with, prevents that attention to them, or even their remark. Who amongst us, for instance, has noticed the existence of what are called positive and negative after images, after looking at some strongly illuminated object, or would have gauged the fact that a certain portion of the nervous system can be fatigued by a colour, and give rise to images of its complementary, had not an enterprising advertiser, who manufactures a household necessary, drawn attention to it in a manner that could not be misunderstood.

If on an autumn afternoon we pass through a garden whilst it is still perfectly light, we can notice the gorgeous colouring of the flowers, and appreciate with the eyes the beauty of each tint. As evening comes on the tints darken, the darkest-coloured flowers begin to lose their colour, and only the brightest strike the eye. When night still further closes in every colour goes, though the outlines of the flowers may still be distinguished; and it would not be impossible, in some parts, to see a tiny speck of pale light upon the ground amongst them. This speck of light we should know from experience to be the light from a glow-worm. Why is it that we lose the colour of the flowers and recognize the tiny light from this small worm? The reason for the one is that in order for objects which are not self-luminous to be seen at all, light must fall on them and illuminate them, and the light which they reflect may be coloured if they possess the qualities to reflect coloured light. The glow-worm's light is seen, not because it does not emit light in the day-time, but because the eye, being limited in sensitiveness, is unable to distinguish it when it is flooded with the light of day. The glow-worm, however, is self-luminous, as is shown by the fact that it emits light in the dark, the light itself being slightly coloured if compared with that of day. That a candle-flame or the sun is self-luminous is an axiom, and need not be philosophised upon; but what must be impressed on the reader is, that though an object which requires to be illuminated to be seen, is not self-luminous, yet when illuminated it does in fact become a source of illumination to the eye, although the light is only light reflected from its surface. It is a point worth remembering that the rougher the surface of an object, the brighter to the eye it will be. That is, a coloured object when polished will be a bad secondary source of illumination, as the light incident upon it will be very nearly reflected from the surface, according to the ordinary laws of reflection; but if it be roughened it will become a much better source, as the roughnesses, though obeying the laws of reflection, will reflect light in every direction. A good example of this is an ordinary sheet of glass. Light from a source falling on its surface is scarcely reflected in any direction except in that determined by the ordinary laws of reflection, and it will be scarcely visible to the eye. Grind its surface, however, and the innumerable facets caused by the grinding will reflect light back to the eye in whatever position it be placed, and will thus be distinctly seen.

We may here premise that even the roughest surface will reflect a greater percentage--varying greatly according to the nature of the surface--of light in the direction which it would do if it were a smooth surface than in any other; and in taking measurements of the light irregularly reflected from a rough surface, this fact must be borne in mind.

Not only must we know how colour is produced, but we must also be able to refer it to some standard which shall be readily reproduced, and which shall be unalterable. There are two variable factors which have to be taken into account in colour experiments: the first is the quality of light which illuminates the object, and the second is the sensitiveness of the eye which perceives it, as light is only a sensation which is recognized by the brain through the medium of the eye. We shall, as we go on, see that different qualities of light may cause objects to appear of different hues, and further that eyes may vary in perceptive power, to an extent of which the large majority of people are not aware. Hence it becomes necessary as far as possible to eliminate these variables.

The task which we have set ourselves to perform then, is first to find a suitable light for experimental work, and next to endeavour to refer colour to an eye which has no abnormal defects. This being accomplished, we have then to find means to measure the different constants which are involved in colour, and to refer the measurements to some standard. Colour constants are three, viz. hue, luminosity, and purity; and it will be seen that if these three are determined, the measurement of the colour is complete.

Perhaps the meaning of these terms may require to be explained. The hue of a colour is what in common parlance is often called the colour. Thus we talk of rose, violet, magenta, emerald green, and so on, but for measuring purposes the hue had best be referred to the spectrum colours as a standard (the means of doing so will be shortly explained), for they are simple colours, which can be expressed by numbers. Compound colours, which it may be said are invariably to be found in nature, being mixtures of simple colours, can be just as readily referred to the spectrum. By the luminosity of a colour we mean its brightness, the standard of reference being the brightness of a white surface when illuminated by the same white light. By the purity of a colour we mean its freedom from admixture with white light. An example of different degrees of purity will be found in washes of water-colours of different tenuity. Thus if we wash a sheet of paper with a light tint of carmine, the whiteness of the paper is not obliterated; if we pass another wash over it the whiteness of the paper is lessened, and so on. The lightest tint is that which is most lacking in purity.

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