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THE CHEMICAL ACTION OF LIGHT 45
from a drawing is often blemished by little iron-mould specks in the paper invisible to the eye. These specks frequently appear as black points. There are faces with little yellow specks that do not strike the eye, but which come out very dark in photography. Some years ago a lady was photographed at Berlin, whose face had never presented specks in a photograph. To the surprise of the photographer, on taking her portrait specks appeared that were invisible in the original. A day later the lady sickened of the small-pox, and the specks, at first invisible to the eye, became then quite apparent. Photography in this case had detected before the human eye the pock-marks, which were, doubtless, slightly yellow.
In the photographs of paintings, if ordinary plates are used, such abnormal action of colour becomes still more evident, and can only be removed by appropriate retouching.
It is proper to observe, however, that by no means all shades of blue become light in photography. For example, indigo forms an exception, appearing as dark as in nature. The reason of this is, that indigo contains a considerable amount of red. On the other hand, cobalt blue and ultramarine produce almost the effect of white. Again, vermilion becomes dark, also English red ; whereas Turkey red, which contains blue, becomes very light. Chrome yellow becomes much darker than Naples yellow ; Schweinfurt's green becomes lighter than cinnabar green. No one of our pigments is a perfectly pure spectrum colour, but consists always of a mixture of different colours, and its light value therefore is essentially modified in photography.
If the effect of the colours of the spectrum on ordinary photographic plates is more narrowly examined, it is observed that the indigo produces the greatest action. Nevertheless, the differently sensitized photographic plates offer somewhat various results in this respect. Chloride of silver is most sensitive to violet, but non