Professor John Trowbridge delivered a lecture last night in the Jefferson Physical Laboratory, under the auspices of the Engineering Society, on "Cathode Rays." The large lecture room was filled to its utmost capacity long before the lecture began and every avilable spot was taken.
"The public," said Professor Trowbridge, "is not generally interested in scientific experiments, but there is something so mysterious about the invisible rays that it is no wonder their interest has been aroused. The rays pass through boards, ebonite, thin metals, and penetrate the inmost parts of the body and yet they do not pass through a window pane."
Professor Trowbridge confined his lecture to the practical application of Roentgen's discovery to the arts and especially to surgery. He called attention to the methods by which use was made of the rays and the apparatus, and dwelt especially upon the fact that a photograph can be taken with simple means in reach of all.
Cathode means the way down or the way out as distinguished from the annode, the way in. Taking an ordinary Leclanche cell, such as is used in connection with house bells, Professor Trowbridge showed how the carbon pole would be the annode and the zinc pole the cathode. It would take 50 or 60 of such cells arranged in series of tandem to produce one incandescent light. If the loop of wire is broken in the middle the lamp will go out. Then if more cells are added until the number is 10, 000 a pale luminous glow will come from the lamp. One terminal of the lamp is called the annode and from this stream these invisible rays.
In the accounts of experiments mention is made of Crooke's tube. As the lecturer showed, it is nothing more than a large receiver of air from which the air can be exhausted. In order to get the rays, however, you must push the exhaustion only to a certain point. Now instead of using the 10,000 cells already mentioned to produce the pale blue flame, a Ruhmkoff coil is introduced, which makes it possible to get a high electro-motive force.
The Ruhmkoff coil consists of two coils; one inside called the primary which is a piece of wire only a few feet long; another on the outside called the secondary which is about a mile long. By passing a strong current through the shorter wire and breaking the current a great electric stress and a high force is obtained. Approximately the 10,000 cells would produce 10,000 volts but the three Ruhmkoff coils used in the experiments last night produced approximately 50,000 volts.
By a transformation of the Ruhmkoff coils in series and connecting the coils with the tube and then exhausting the air, you get the cathode rays. At this point the experiment was performed. The light in the Crooke's tube was a pale blue on the two pole and light pink in the centre. A fluorescence was given off which was of a light blueish-green color. This, Professor Trowbridge explained, was thought by some to be the cathode rays, but the point is still in doubt. No one knows what the rays are. In connection with this the lecturer took up a large slim beaker filled with kerosene, which is generally colorless, but when held near the lighted tube the kerosene was of a light blue color.
After the experiment was finished Professor Trowbridge explained that the same light can be produced with a common electric machine, but the plate must be exposed an hour when this machine is used. This shows how simple the process really is. The chief advantage of the large apparatus is that when it is used only a minute's exposure is necessary. The distinguishing character of the cathode rays is that they do not go between the two poles. They can be thrown any way. For instance, a small piece of aluminum can be used to throw them in any direction.
Coming now to the method of taking photographs, the lecturer showed the difference in the process. In taking a photograph with cathode rays a plain dry plate is used. There are the customary slides in the holder but there is no central opaque partition. The hand or object to be photographed is put on the slide and you get a photograph of the shade. As glass absorbs the rays a lens would be of no use and would prevent the taking of a photograph. Thus the common process of photographing is exactly reversed with the cathode rays. This was illustrated by a picture thrown on the screen where a camera was shown with the larger end towards the cathode and the lens pointed away uselessly.
Cathode photographs have been called shadow pictures, but this is not an exactly correct name for them. This is because a piece of glass as thick as a piece of paper absorbs a large number of rays and throws a very thick shadow on the plate while even a thick piece of wood throws hardly any shadow. The shadow photograph, however, is very exact in other respects. In the photograph of the human hand it shows the gradations of the absorption of the rays with the thickness of the bones.
The rays will go through sheets of aluminum 1-10 of an inch thick; a greater thickness of aluminum will absorb only a portion of the rays. The glass of the Crooke's tube is only 1-60th of an inch thick, but even glass as thin as this absorbs so many rays that it presents great obstacles. It has been suggested that an aluminum window be put in the tube and that the photographs be taken with the rays that come through the aluminum, for the reason that the aluminum absorbs hardly any rays while the thinnest glass absorbs an enormous number of them. The objection to the aluminum window is that the atmospheric pressure of 15 pounds to the square inch would break the tube.
Celluloid tubes and tubes of wood soaked in some substance making it air tight have also been suggested. The objection to these are that the wires entering through these substances get heated and heat everything we know of except glass.
Doubtless the rays exist in common electric lights and in sunlight, but as yet scientists have proved their existence only in a vessel such as Crooke's tube. With these invisible rays photographs can be taken at any time, even in the sunlight. In connection with the cathode photographs it is interesting to note the length of exposure that has been necessary to take photographs at different periods. The table is as follows:
Heliography, 1827, 6 hours.
Daguerreotype, 1839, 30 minutes.