712 REPORT—1896, 
development is observed and measured in terms of the width of a tooth. If the 
speed of rotation is known, the length of time the effective radiation persists can 
be at once deduced. 
A mercury brake worked simply by hand was generally used. The tube was 
distant from the plate about eight centimetres. 
When the wheel is rotated sufficiently fast, a drawing out of the image is 
always observed ; but the amount of this drawing out in each case is found to vary 
in an important way with circumstances, and is probably but a measure of the 
length of time the E.M.F. remains above the value necessary for discharge, and 
thus ultimately depends upon the arrangements used—the coil, Ke. 
If a spark gap in parallel with the tube be provided, the drawing out is cut 
short by the passage of a spark at the gap. How early this occurs depends on the 
distance between the sparking points. In this way comparatively sharp-looking 
images are obtainable without otherwise altering the arrangements. 
The time the radiation lasted, as measured from the photographs obtained in 
this way, varied from, roughly, the g$>5 to the zo5$95 of a second. In the first 
case the points were too far apart for a spark to pass; in the latter the points were 
as near as possible consistent with getting any photographic effect. 
Experiments can also be made by using a phosphorescent screen, but the 
measurements are not capable of being made with the same certainty ; however, 
it is a more convenient way to demonstrate the existence of the early cut-off in the 
duration of the radiation caused by a parallel spark. 
When the brake of the primary is made by hand by means of the usual hammer 
arrangements, the results sometimes are difficult to explain. Often three images 
appear as if three sparks occurred, each image being drawn out. This might be 
merely due to something oscillatory in the circuits, but for the fact that the character 
of the drawing out is peculiar, the half shadow region shading the wrong way. 
That is to say, instead of passing uniformly in shade from the longer exposed parts 
to the parts always covered while the radiation lasted, there is a fluctuation in 
intensity, so that a tooth is bounded first by a dark line or band, while the region 
longer exposed outside this is not so black. 
6. On the Relations between Kathode Rays, Réntgen Rays, and Becquerel 
Rays. By Professor Sirvanus P. Toompson, /.2.S. 
The author described experiments, made with vacuum tubes of several shapes, 
to test several points in the relations between the various kinds of rays. It was 
found that when kathode rays were caused to fall on an oblique platinum piece in 
the interior of the tube, true kathodic shadows could be obtained in the rays 
reflected from the platinum surface, from metallic and other objects interposed 
between this target and the walls of the tube. These shadows were deflected 
by magnets, and were affected in size by electrifying the interposed objects, At 
the same time, and when the tube was sufficiently highly exhausted, Rontgen-ray 
shadows were obtained on a luminescent screen outside; but these shadows, unlike 
the shadows of the reflected kathodic rays within the tube, were not deflected by 
either magnetic or electrostatic influences. Experiments on filtermg the kathodic 
rays, direct and reflected, through screens of aluminium of various thicknesses 
showed that the more deflectable rays were more easily stopped by screens than the 
less deflectable ; and that the power of producing luminescence in different bodies 
differed for rays of different deflectability. Uranium, as a target, appeared to be 
more active than platinum in evoking emission of Réntgen rays, 
