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44 THE ADVANCE OF PHOTOGRAPHY
violet for a space almost as long as the whole visible spectrum.
From this fact the existence of the ultra-violet rays was ascertained. The retina of our eye and the photographic plate possess an entirely different sensitiveness. Our eye is affected most powerfully by yellow and green light. These colours appear to us the brightest, while the photographic plate is only slightly affected by them ; on the other hand, indigo and violet rays, which appear dark to our eye, and even rays which to our eyes are invisible, produce a powerful action on the plate.
It is natural, therefore, that photography should represent many objects in a false light. Photography is much less sensitive than the human eye to feebly lighted objects. This is most clearly seen in the fact that the eye can easily perceive objects by moonlight which is 200,000 times weaker than that of the sun ; whereas the old wet photographic plates were not able to produce any picture of a moonlit landscape, and even the more highly sensitive dry plates require a relatively long exposure for this purpose. The early photographic landscapes by moonlight which were offered for sale were taken in daylight and copied very darkly, so that they produced the effect of moonlight. Such pictures were at one time very popular in Venice.
This small sensitiveness of the ordinary photographic plate to feeble light explains the reason why shadows are generally too dark in photographs. To these defects must be added the false action of light, — blue generally appears light coloured, yellow and red, black. Yellow freckles appear, therefore, in a picture as black spots, and a blue coat becomes much too light. Blue (and therefore dark) flowers on a yellow ground produce, in photography, too light flowers on a too dark ground. Red and also fair golden hair becomes almost black. Even a very slight yellow shade has an unfavourable effect. A photograph