PENETRATES SOLIDS.

Professor Trowbridge, director of the Jefferson Physical Laboratory obtained on Wednesday afternoon a distinct impression upon a photographic plate by means of the Professor Rontgen cathode rays acting through wood and pasteboard. The impression has been fixed and is capable of giving a print upon ordinary blue print or other sensitive paper.

A very sensitive Cramer dry-plate about four inches long and 1.5 inches wide, was put, film side up, into a wooden box, having a close-fitting sliding wooden cover. Upon the sensitive plate were laid two clear glass slips, less than one sixteenth of an inch thick. A space was left between them about four inches long and one half an inch deep. Across the glass slips to hold them in place was put a narrow bar of pine wood five-sixteenths of an inch thick. The wooden cover, three-sixteenths of an inch thick, was then pushed into place. The wooden box thus prepared was placed within a covered pasteboard box, the walls of which were about one thirty-second of an inch thick. The pasteboard box, with its contents, was placed one or two inches from the brightly fiorescent part of an ordinary spherical Crookes tube, and the action was maintained with this arrangement about two minutes, when the tube became so hot that the operation was stopped. The sensitive plate was then taken out and then developed with rodinol. Soon the part which had not been shielded by the glass slips began to show dark, and in a very short time the development was completed, the boundaries of the exposed part of the plate being well defined for the whole length of the plate. The image was then "fixed" in the ordinary way.

This was the second attempt made by Professor Trowbridge to obtain the desired effect with a Crooke's tube. The first attempt, made a few minutes before, was partly successful. The ordinary sixty-volt alternating current used for lighting the building was sent through the primary of an ordinary coil. The resistance of the primary of this coil is one-tenth of an ohm, and of the secondary 6000 ohms. The current through the primary was not stronger than 15 ampheres. The current from the secondary of the first coil was sent through the primary of 25 turns of a Tesla induction coil, the secondary of which has 500 turns. The secondary will give a spark through about six inches of air. Its terminals were connected with the electrofloe of the Crooke's tube already mentioned.

The current thus furnished to the coil being an alternating one, the appearance of the tube during action was somewhat different from that ordinarily described. The fluorescent effects were less sharply localized than usual. From the ease with which the photographic effect was obtained, it appears doubtful whether so complicated and powerful electrical apparatus was really necessary. It happened to be at hand and was therefore used. It is evident that the impression obtained on the plate is rather a print than a negative.

Whatever the cause is that produced the effect, it certainly worked through a thickness of wood which at one place was not less than one-half an inch. At other places the thickness of the wooden shield was only about one-eighth of an inch, but it is very difficult to distinguish on the plate the part that was covered by the extra thickness. It is evident that an effect would have been produced through more than one inch of solid wood.