[Illustration: THOMAS A. EDISON EXPERIMENTING WITH THE RÖNTGEN RAYS.]
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.