136 TRANSACTIONS OF THE [MAR. 2, 
‘light reflected from the mirror through the hole falls upon the 
center of the mirror attached to the base of the instrument; 
then starting the clock the instrument will keep the beam ina 
constant position. Prof. Hallock in discussing the paper called 
attention to the accuracy with which the heliostat operated and 
related his experience with a very large one-mirror heliostat in 
the Smithsonian Institution at Washington, which, however, was 
thoroughly unsatisfactory. Prof. Woodward and Prof. Jacoby 
also entered into the discussion of the relative merits of the 
various heliostats, especially the typical one-mirror and two- 
mirror types. 
Prof. M. I. Pupin then brought before the Academy some re- 
cent observations he had made while experimenting with 
X-rays. In the first place he pointed out that many Crook’s 
tubes after a certain amount of use had their vacuum improved, 
so that the induction spark passed outside the tube rather than 
through it. Prof. Pupin was at a loss to altogether sat- 
isfactorily explain the cause, but believed that it might possibly 
be due to the condensation of some of the gas remaining in the 
tube, and explained several experiments which he had made 
already confirming the observation that the vacuum was im- 
proved with use, and that in proportion as the vacuum im- 
proved the tubes were better for X-ray photography. Another 
of the phenomena observed by him was that in developing the 
photographic plates the development began at the glass side of 
the film and not at the outside of the film, leading to the inference 
that the rays penetrated the film and rendered the glass fluores- 
cent, this fluorescent light then acting upon the film. Following 
' the suggestion of this observation he painted the inside of a box 
with platinum-barium-cyanide and laid a photographic plate 
against it, making a photograph then through the sides of the 
box. The X-rays develop the fluorescence in the cyanide and 
then penetrate the plate. He obtained very good results with 
much shorter exposures than by the original method. 
On motion the meeting then adjourned. 
J. F. Kemp, 
Secretary. 
