In the case of a coloured powder or of a matt surface, the white light is scattered without change of colour approximately equally in all directions, so that it becomes impossible to see the true colour of the substance. This effect would be diminished by wetting the surface of the body with water (it is known that photo graphic proofs printed on matt papers are always more beautiful and vigorous when wet than after drying), but this kind of colour intensification can be made more permanent by replacing the water, a volatile liquid, by a varnish, the effect of which will be all the more marked if the colour is contained in a more refracting medium. This is the reason why oil paintings or colour suspensions in gelatine give much more intense colorations than can be obtained by means of water colours in a medium of a very weak proportion of gum arabic, or by pastels, a medium-free coloured powder, of which the character of lightness disappears when it is treated with a fixative, i.e. covered with a varnish.
8. The colours given in the table in § 5 are pure saturated colours. On the other hand, the absorption band of a colour, i.e. the spectral region in which there is more or less complete extinction of the rays of the incident white light, may be of any extent, and its absorption may be complete or only partial. If the absorp tion band extends throughout the whole spec trum, either a dark or broken hue will result (corresponding respectively with dilution of the pure saturated colour with black or grey) according as the absorption in certain regions is complete or not. If the absorption band is of limited extent, and if no rays are completely extinguished, a pale hue will result, equivalent to mixing white and the corresponding saturated colour.
Thus, for example, pure saturated orange absorbs the blue-violet completely and an appreciable fraction of the green, whilst trans mitting freely all the red. If the absorption is incomplete, with a maximum in the blue-green, the orange will give a flesh colour due to dilution with white.
In like manner, the admixture of black or of grey with orange gives, in the former case, a dark terra-cotta, and in the other, a broken cream colour.
All shades of colour can be accurately defined by means of the curve showing in the normal spectrum the percentage of diffused light for each of the radiations.
9. Nearly all objects, whether natural, dyed, or painted, even when they appear of very pure and intense hue, give actually only deep or broken hues, since no radiation is reflected completely.
The purest yellows, oranges, and reds reflect generally about 7o per cent of the radiations which, theoretically, they ought to reflect (or scatter) completely. This proportion drops to about 2o per cent for the blues and violets and is less than 15 per cent for the greens. In this connection some measurements carried out by A. J. Bull (1911) on different leaves are given in the table below— This explains the difficulties that are always experienced in giving a satisfactory rendering of leaves.
io. Absorption by Coloured Transparent Media. These phenomena are much more definitely shown when the light is filtered through a coloured transparent medium, because the change which takes place in the light during its passage through the medium is not, as in the case of coloured surfaces, viewed by the light which they reflect, masked by the white light reflected from the surface.
Let it be remembered, first of all, that, con trary to a widespread belief, the light is not coloured by its passage through a coloured medium ; it only appears coloured because during its passage through the medium certain of the radiations which constitute white light are absorbed. A light-filter 1 always transmits less than it receives ; even the radiations which are transmitted most completely by the most perfect filter are slightly weakened, to an extent not less than 5 per cent.
The use of light-filters is the simplest method which can be used to obtain a coloured light of any desired quality. They are constantly used in photographic practice for such purposes as lighting dark-rooms, correcting the colour ren dering by photographic plates, etc.
A light-filter, like a pigmentary colour, is defined by its absorption band. It is as easy to obtain light-filters in gelatine identical with one another by using suitable quantities of pure colouring matters, as it is difficult (it would be even more exact to say impossible) to obtain identical coloured glasses of different makes. When it is also stated that, as a rule, the coloured glasses available for photographic uses are taken from among those used for making stained glass windows or for railway signals, it will be seen how difficult it is to obtain results which are even roughly in agreement by using glasses specified only by their colour. The im portance of the progress which has been made since the beginning of this century in replacing coloured glasses by scientifically-determined filters can easily be realized.