Dark Light.

The claims of M. Gustave Le Bon, a French physicist, to have discovered what he terms "dark light" — an invisible form of radiation arising from the passage of ordinary light through apparently opaque metal plates, and capable of producing shadowgraphs like those of the X rays — have already been noticed in these columns. M. Le Bon's experiments have been described before the French Academy of Sciences, a sufficient proof that they have been performed in good faith; but his conclusions have been usually looked upon with suspicion, men of science having generally thought that he has been deceived by the filtration of light through chinks in his plate holder, or by some similar action. Nevertheless, his experiments and papers continue, and in his last paper, read on May 11, he asserts that many forms of invisible radiation. including Roentgen's X rays, rays from fluorescent bodies, the rays discovered by himself as noted above, and others, are all merely forms of "dark light," which term he thus widens in application to cover all kinds of radiation capable of affecting a photographic plate, but not the retina of the eye. Le Bon's experiments have been unaccountably neglected in this country. They are so simple that it should be easy for almost any one to prove the truth or falsity of his claims, yet no one apparently has thought it worth while to take the trouble. One American scientific magazine of high repute even refers to his experiments as experiments on X rays, says the Literary Digest. It is quite certain, however, that there are forms of invisible radiation capable of taking a photograph, besides the famous X rays. Some of the methods of photographing "in the dark" were known, in fact, long before Roentgen's discovery. It may be that Le Bon has really discovered a new and related form at radiation, and in any case his recent classification of all such forms under one comprehensive head is a step in the right direction. We translate below an extract from the paper alluded to above, together with an abstract of other portions of it, from Cosmos (Paris, May 23):

"To place his previous experiments definitely beyond all the objections brought against 'dark light,' notably that which suggests the filtering of ordinary light through the cracks of the plate holder, M. Le Bon has undertaken new experiments with the object of condensing it on the surface of metal plates, and then obliging it to pass through these and set on photographic plates in darkness. We quote from his communication the description of his experiments:

"'Take a sheet of copper and one of lead, about one millimeter in thickness; place each of these two sheets in a photographic printing frame instead of the sheet glass and expose one of the faces — only one — at a distance of 20 centimeters [8 inches] to the light of an electric arc, for one hour. Remove the two frames to darkness and allow them to cool for two hours. Remove the sheets from their frames; then, between the two faces that have not been exposed to the light, place a sensitized glass plate, and the object that we wish to reproduce, a photographic negative for instance, taking care that the object shall be between the copper and the sensitized plate. To avoid all contact action, be careful to separate the sensitized glass from the object to be reproduced, by a sheet of glass or celluloid. It will be sufficient to leave the whole in darkness for five or six hours, to obtain on development a perfect image of the object placed between the metal sheet and the photographic plate. It is then evident that the light condensed on one of the faces at the sheet or copper has traversed the metal and made an impression on the photographic plate.'

"Adopting the term 'dark light' for all terms of invisible radiation hitherto discovered, M, Le Bon believes that he has been able to establish the following classification:

"X Rays. — These traverse black paper and organic substance, do not pass through most metals, and are neither reflected nor refracted.

"Invisible Rays from Fluorescent Bodies. — These pass through metals, as Messrs. D‘Arsonval and Becquerel have shown, are refracted and reflected, and present, consequently, no peculiarity permitting us to identify them with the X rays.

"Rays that are Formed when Visible Light Falls on Metallic Surfaces. — The researches of M. Le Bon show that these rays do not pass through black paper, nor through the greater part of organic substances, but that they pass through a large number or metals. They also possess the property of being condensed and diffused, like electricity, on the surface of metals.

"Rays Belonging to Organic Beings. — Rays are emitted by organized beings in darkness, which allow us to photograph them, as M. Le Bon has shown by operating on ferns, fishes, and various animals. These rays appear to be related to the invisible rays of phosphorescence, but they differ nevertheless in that they do not pass through metallic bodies, at least those experimented upon — notably aluminum."

Originally published in Scientific American 75:3 (July 18, 1896), p. 41.

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