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Mr. A. A. C. Swintoii. 



rays accurately upon the carbon, a small bright spot appears at first ; 

 as the vacuum goes down this point becomes larger and fainter, but 

 still solid. Suddenly it becomes hollow and brighter; then, as the 

 vacuum falls still further, the ring becomes solid again, though 

 larger and more faint than before ; finally it disappears. After this 

 stage it can be reproduced momentarily, without alteration to the 

 vacuum, by switching the magnet on and off suddenly, when it is 

 usually hollow, but sometimes solid. 



The Convergent Gone at Higher Vacua. 



As has been mentioned, the carbon anti-cathode screen was found 

 useless for investigating the convergent cone of cathode rays at 

 anything but a very low vacuum, by the reason of the well-known 

 difficulty in getting any discharge to pass when the distance between 

 the electrodes is less than the thickness of the dark space, and for the 

 further reason that if the anti-cathode screen was not connected to the 

 anode, it became itself negatively charged and acted as an additional 

 cathode when brought into the space between the cathode and the 

 focus. 



Under these circumstances, it was thought that possibly some 

 additional information might be obtained with regard to the form of 

 the convergent cone at high vacua, by making the concave cathode 

 itself of carbon. A tube was therefore constructed having a concave 

 carbon cathode, the diameter of which was 1 inch, and the radius 

 of curvature 0*75 inch. The appearance of the cathode with this tube 

 is shown for a fairly high vacuum in fig. 20, in which the cathode 

 itself is shown in section, so as to let the form of the discharge be 

 better seen, As will be observed under this condition of vacuum, 

 which was too high to show any divergent cone, the cone of conver- 

 gent rays appears to be contracted in diameter at its base, and to come 

 off from the central portion of the cathode only, the remaining 

 surface of the cathode being apparently inactive. This was found to 

 be still more the case at higher vacua, as will be seen from fig. 21, 

 which shows in a similar manner the form of the cathode discharge 

 in a tube exhausted to a very high vacuum. In this case, as will be 

 observed, the whole of the cathode rays appear to come off from a 

 very small spot in the centre of the cathode. Further, that this 

 small spot is, at any rate, the source of most, if not all, activity, was 

 evident from the fact that it became luminescent exactly in the same 

 manner, but in a less degree, than had previously been observed with 

 a carbon surface upon which cathode rays were concentrated. 

 Whether this surface luminescence of the cathode carbon at the point 

 where the cathode rays leave it is due to the violent tearing away of 

 particles of carbon, or to some other cause, it is difficult to say, but 



