Dr. Robb has made a series of novel and important experiments with tubes from which the air has been exhausted in varying degrees, and has concluded from these that it is impossible to produce the Röntgen phenomena unless there is present in the tube an almost perfect vacuum. Through a tube half exhausted, on connecting it with an induction coil, he obtained merely the ordinary series of sparks; in a tube three-quarters exhausted, he obtained a reddish glow from end to end, a torpedo-shaped stream of fire; through a tube exhausted to a fairly high degree—what the electric companies would call "not bad"—he obtained a beautiful steaked effect of bluish striæ in transverse layers. Finally, in a tube exhausted as highly as possible, he obtained a faint fluorescent glow, like that produced in a Crookes tube. This fluorescence of the glass, according to Dr. Robb, invariably accompanies the discharge of Röntgen rays, and it is likely that these rays are produced more abundantly as the fluorescence increases. Just how perfect a vacuum is needed to give the best results remains a matter of conjecture. It is possible, of course, as Tesla believes, that with an absolutely perfect vacuum no results whatever would be obtained.
Dr. Robb has discovered that in order to get the best results with shadow pictures it is necessary to use special developers for the plates, and a different process in the dark-room from the one known to ordinary photographers. In a general way, it is necessary to use solutions designed to affect the ultra-violet rays, and not the visible rays of the spectrum. Having succeeded, after much experiment, in thus modifying his developing process to meet the needs of the case, Dr. Robb finds that he makes a great gain in time of exposure, fifteen minutes being sufficient for the average shadow picture taken through a layer of wood or leather, and half an hour representing an extreme case. In some shadow pictures, as, for instance, in taking a lead-pencil, it is a great mistake to give an exposure exceeding two or three minutes; for the wood is so transparent that with a long exposure it does not show at all, and the effect of the picture is spoiled. Indeed, Dr. Robb finds that there is a constant tendency to shorten the time of exposure, and with good results. For instance, one of the best shadow pictures he had taken was of a box of instruments covered by two thicknesses of leather, two thicknesses of velvet, and two thicknesses of wood; and yet the time of exposure, owing to an accident to the coil, was only five minutes.
Dr. Robb made one very interesting experiment a few days ago in the interest of a large bicycle company which sent to him specimens of carbon steel and nickel steel for the purpose of having him test them with the Röntgen rays, and see if they showed any radical differences in the crystalline structure. Photographs were taken as desired, but at the time of my visit only negative results had been obtained.
Dr. Robb realizes the great desirability of finding a stronger source of Röntgen rays, and has himself begun experimenting with exhaustive bulbs made of aluminium. One of these he has already finished, and has obtained some results with it, but not such as are entirely satisfactory, owing to the great difficulty in obtaining a high vacuum without special facilities.
I also visited Professor U.I. Pupin of Columbia College, who has been making numerous experiments with the Röntgen rays, and has produced at least one very remarkable shadow picture. This is of the hand of a gentleman resident in New York, who, while on a hunting trip in England a few months ago, was so unfortunate as to discharge his gun into his right hand, no less than forty shot lodging in the palm and fingers. The hand has since healed completely; but the shot remain in it, the doctors being unable to remove them, because unable to determine their exact location. The result is that the hand is almost useless, and often painful.
Hearing of this case, Professor Pupin induced the gentleman to allow him to attempt a photograph of the hand. He used a Crookes tube. The distance from the tube to the plate was only five inches, and the hand lay between. After waiting fifty minutes the plate was examined. Not only did every bone of the hand show with beautiful distinctness, but each one of the forty shot was to be seen almost as plainly as if it lay there on the table; and, most remarkable of all, a number of shot were seen through the bones of the fingers, showing that the bones were transparent to the lead.
In making this picture, Professor Pupin excited his tube by means of a powerful Holtz machine, thus following Dr. Morton in the substitution of statical electricity for the more common induction coil.
Professor Pupin sees no reason why the whole skeleton of the human body should not be shown completely in a photograph as soon as sufficiently powerful bulbs can be obtained. He thinks that it would be possible to make Crookes tubes two feet in diameter instead of a few inches, as at present.
Thomas A. Edison has also been devoting himself, with his usual energy, to experiments with the Röntgen rays, and announces confidently that in the near future he will be able to photograph the human brain, through the heavy bones of the skull, and perhaps even to get a shadow picture showing the human skeleton through the tissues of the body.