At an early period of the study of photography, it was observed that the red, orange and yellow rays are endowed with antagonistic powers, preventing and destroying the action produced by white light, or by the rays properly called
photogenic rays
. One of the first discoverers of this property was Dr. Draper of New York: his experiments were made with the pure rays of the spectrum acting on the Daguerreotype plate. Previously to this, however, Sir J. Herschel had made similar observations on the action of the pure rays of the spectrum on several kinds of photogenic paper. Dr. Draper also found that the red, orange and yellow rays which protect the plate from ordinary photogenic action, are themselves capable, when isolated, of producing a peculiar photogenic effect. In opposition to the hypothesis of an antagonistic or destroying action exercised by the red, orange and yellow rays, M. E. Becquerel announced that those rays are endowed with the property of continuing the action commenced by the photogenic rays. The author of the present paper has made a series of observations on light transmitted through certain colouring media, through the vapours of the atmosphere, and through red, orange and yellow glasses. Having directed a camera obscura to the sun when his disc appeared through a fog quite red, he obtained, after ten seconds, a black image of the sun. The red sun had produced no photogenic effect, although the surrounding spaces had been sufficiently affected by the photogenic rays coming from the zenith to attract the white vapour of mercury; thus proving that the red rays have no photogenic power. In another experiment he left the plate in the camera during twenty minutes. The sun had passed over a long space on the surface of the plate, and the result was a long image of the sun, quite black throughout; so that not only the red sun had produced no photogenic action, but the red rays had destroyed the effect produced previous to their passage. Not content with the result obtained by the slow motion of the sun, he next moved the camera obscura from right to left, and
vice versâ
, lowering it each time by means of a screw. In this manner the sun was made to pass rapidly over five or six zones of the plates, and its passage was marked by long black bands, while the intervals were white; showing again that in order to destroy the action of the photogenic rays, it was sufficient to cause the red rays to pass rapidly over the spaces previously affected by the former.
Marsilio Ficino's Allegorical Reading of Optical Phenomena
