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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. Kœnig'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

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COLOUR MEASUREMENT

AND

MIXTURE.

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CHAPTER I.

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