. The evolution of forces . estroy the impression on the plate produced by theilluminating rays. Let us place in a photographic dark slide a plateof ebonite Avhich may be 1 millimetre thick, butwhich it is better to reduce to i millimetre, so asto obtain sharp outlines. On or under this plate, wowill glue a cross cut out of a sheet of tin. Behindthe ebonite plate I will introduce, in the dark, agelatino-bromide plate of very fine grain, previouslyclouded by exposing it for two seconds to the lightof a candle. I close the frame and expose it forone hour in sunlight. The infra-red rays passthrou
. The evolution of forces . estroy the impression on the plate produced by theilluminating rays. Let us place in a photographic dark slide a plateof ebonite Avhich may be 1 millimetre thick, butwhich it is better to reduce to i millimetre, so asto obtain sharp outlines. On or under this plate, wowill glue a cross cut out of a sheet of tin. Behindthe ebonite plate I will introduce, in the dark, agelatino-bromide plate of very fine grain, previouslyclouded by exposing it for two seconds to the lightof a candle. I close the frame and expose it forone hour in sunlight. The infra-red rays passthrough the ebonite and destroy the cloud producedon the plate by the previous exposure. In the partprotected by the metallic cross they will not haveacted, and that is why, on developing the image,we shall find a reproduction of the metal cross on avery light ground. The ebonite plate may also be 340 THE EVOLUTION OF FORCES replaced by two or three superposed sheets of blackpaper. There is no analogy between this experiment and. Fig. 40.—Dcraonstration, hi/ithcantar/onistic properties of the two endsof the spectrum, of the transpareney of opaque hodics {black paper,ebonite, <fcc.) to invisible light, and of the opacity of the same bodiesto visible light. To show the transparency of bodies to theinvisible rays, we take as our basis the property possessed bythese last of destroying photographic impressions. In a darkslide is placed a plate of ebonite with metal stars glued in themiddle of it. We place above it a sensitive plate, of which onehalf has been clouded by two seconds exposure to the light ofa candle. The slide is then exposed for one hour to developing the plate no trace of an impression is perceivedon the unclouded part. On the clouded part, the infra-redrays have passed through the ebonite and have destroyed thecloud, save under the metal star. It follows from this that thislast part is kept back, and consequently appears in black onthe negative image and
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