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 Roentgen 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 Roentgen 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 Roentgen 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 una