
Photographic Action of Radium Rays. 291 
of lightning-flashes which are obtained when several images 
are taken on one plate, or when images are obtained of 
lightning-flashes with a bright cloud behind them. 
“Tn my experiments an Ilford plate (red label) was exposed 
in a camera toa series of Wimshurst machine sparks, the plate 
being shifted between each spark. Then the plate was placed 
in its envelopes, and a part of it was protected by a thick 
leaden screen. The rest of the plate was exposed to the 
radium. ‘This was repeated for different periods of radium 
exposure. On developing these plates, it was found that 
in those which had been exposed to the radium for only 
short periods, gradually increasing in length, there was a 
progressive elimination of the spark image, but the density 
of the parts corresponding to the radium exposure was 
not so great as that of the parts of the plate only exposed 
to the spark, although the radium action had been able 
to obliterate completely the spark effect. On the other hand, 
when those plates which had been exposed longer, or 
had been exposed to the greater mass of radium, were 
developed, I found that reversed images of the sparks 
appeared. These images were not so wide as the spark 
images. In another case, with still further exposure, I 
observed the radium reversing its own image, and across the 
radium reversed part of the plate there appeared a faint dark 
image of the spark. This might be described asa re-reversal 
of the spark. 
The conditions of these experiments were such that 
probably only the - and y-rays were acting. In the 
Philosophical Magazine for November, 1903, a paper appears: 
by R. W. Wood, on the subject of photographic reversal, 
and in it he says that he has been unable to reverse a spark- 
image by Réntgen rays, which are supposed to be similar, if 
not identical with the y radium rays. If this is the case we 
must ascribe the reversal, which I have described as produced 
by radium, to the action of the §-rays alone. This point, 
however, needs further investigation with screens capable of 
cutting off each class of rays. 
In Wood’s paper he gives what may be called a reversal 
order. It is :-—Pressure-marks, w-rays, Light-shock, Lamp- 
light. In this order each successive influence is ‘able to 
reverse those before it. He also states that Becquerel rays 
will reverse pressure-marks, and that lamplight will reverse 
the effect of Becquerel rays. The Becquerel rays which he 
used were probably those from a compound of uranium. 
My experiments show that Light-shock (from an electric 
spark) may be reversed by Radium rays, and consequently 
