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.