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168 THE ADVANCE OF PHOTOGRAPHY
a yellowish green tint, since it is by this light that one is enabled to see the most with the least amount of light. This is the more convenient, since some Panchromatic plates are less sensitive to green than to any other light.
Orthochromatic plates can be used without a screen, and the results then obtained will compare favourably writh the best of those with ordinary plates. It is, however, for the more difficult subjects that these plates are especially adapted.
They can be used for obtaining " cloud effects," and with a suitable screen, cloud and sky, as well as trees, etc., can be photographed at one and the same time, thus dispensing with the use of cloud negatives.
Then, again, in ordinary landscape effects the results obtained are far more pleasing, for the different shades of green produce different effects upon the plate, and clumps of trees are not represented as a rule by one uniform tint in which a large proportion of the detail is lost to sight.
Perhaps it is in hazy weather that the landscape photographer feels the greatest benefit from these plates. When using ordinary plates any haze renders distant objects very indistinct, and, in fact, a slight fog is sometimes quite sufficient to make it impossible to obtain a distant view. With orthochromatic plates and a suitable screen, while the whole plate may clearly indicate the conditions under which the exposure was made, the distant objects are quite clearly visible.
Even in portraiture such plates will be found useful. The light from some shades of red hair produces an effect upon the ordinary plates very much the same as black hair, and thus such hair appears much too dark in the photograph, while, of course, shades of yellow and red in the clothing will also appear disproportionately dark.
Photography by Invisible Rays of Light. — A very interesting development in the employment of invisible