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.
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