[cox A callendar] experiments on the x-rays 185 



The total eclijDse having- been tidjusted in such ii manner that the slot 

 at the camera end was just on the point of opening at the moment when 

 the other closed, five minute exposures were taken alternately as before 

 at a low and high speed on the two halves of a plate for the space of 

 more than an hour. Both halv(.s of the plate developed perfectly clear. 

 The setting was so fine that if the velocity of the rays had been less than 

 200 kilometres per second, some light must certainly have been restored 

 by the rotation. 



(3) The Method of Partial Eclipse. — The method of total eclipse, 

 if the setting were sufficiently fine, aftbrded perhaps the most delicate 

 test of the velocity of the X-rays. At the same time, it was so far unsa- 

 tisfactor}' that it gave only a perfectly clear plate showing no record 

 whatever of the time and trouble spent in producing it. If the velocity 

 had been measurably small, it could have been determined by this method, 

 either by observing the width of the band of light restored at a given 

 speed of rotation, or by observing the speed required to reproduce the 

 total eclipse at the other side of the slot. To secure this latter result 

 with our apparatus at a speed of 25 revolutions per second, the velocity 

 of the X-rays must have been as low as 7 kilometres per second, or not 

 more than about 20 times the velocity of sound. That we had succeeded 

 in reproducing the eclipse, was a possible, though not a likely, interpre- 

 tation of our failvire to secure any result by the total eclipse method. 

 The intensity of the rays is of course excessively weakened by the dis- 

 tance, and more particularly by the passage through so many fine slots. 

 The failure to aflect the plate might have been attributed to lack of 

 intensity of the rays, or to want of joroper alignment on the focus point. 

 We therefore used the most powerful radiation which we could produce 

 Avithout melting the platinum plate, and we verified the setting of the 

 axis on the focus point both before and after the exposure. 



In repeating the experiment on two subsequent occasions, we adopted 

 the method of partial eclipse. The tube was set so that the near slot had 

 already opened by about half a millimetre or one-third of its width at 

 the moment when the far slot closed. The shadow of the slot obtained 

 in this way on the plate, would be conclusive evidence with regard to 

 the alignment and the sufficiency of the exposure. The velocity of the 

 rays, if measurable small, could also be measured by the widening of the 

 shadow. 



The method of partial eclipse was tried in this manner on two 

 occasions with exposures of upwards of half an hour. The photographic 

 image obtained was a sharp narrow band half a millimetre wide, corres- 

 ponding exactly with the setting of the slot. The two halves of the 

 band, corresponding to the exposures at the high and low speed respec- 

 tively, coincided so exactly that no break could be detected at the point 

 where they met. The edge of the band was so well defined, and the 



Sec. III., 189G. 12. 



