574 HENRY A. Rovv LAND 



when the platinum is red hot from the bombardment, the concave elec- 

 trode being the cathode and a third wire the anode. 



The same tube, with the platinum made an anode and the concave 

 electrode a cathode, produces a profuse radiation of Rontgen rays in all 

 directions on the side of the platinum bombarded by the cathode rays, 

 and none on the other side. In the first case we obtained no rays from 

 the cathode, no rays from the bombarded surface, and only a very weak 

 effect from the anode, indeed almost nothing. Hence the condition 

 for the production of the rays seems to be neither the one or the other 

 but a combination of the two, and we now believe as far as we can yet 

 see that the necessary condition for their production is an anode bom- 

 barded by the cathode discharge. The anode may be, however, an in- 

 duced anode formed on the glass, and the cathode rays may vary a great 

 deal and cease to present the usual appearance of cathode rays. 



Thus, in the best tube that we have, originally made for showing that 

 electricity will not pass through a vacuum, the main source is a point on 

 the end of the anode, where a little point of light appears. Sometimes, 

 across the little interval of 1 mm. between the electrodes, a faint spark 

 or arc crosses from one electrode to the other, and we think that the 

 rays come out especially well under these conditions. Here the action of 

 the bombarding cathode discharge is rather obscure. This little point of 

 light also sometimes appears on the red hot platinum anode men- 

 tioned above, and we have seen it in other tubes, always at the place 

 where Rontgen rays are apparently found. 



Prof. Elihu Thomson has kindly sent us some sketches of tubes hav- 

 ing the anode bombarded by the cathode, and we had previously de- 

 signed some tubes of similar shape, but have not yet found anybody 

 in this country capable of making a sufficiently good vacuum. In many 

 of our best tubes the vacuum is so perfect as to cause a resistance equal 

 to a five or six inch spark in the air. The better the vacuum the 

 greater the number of rays sent out. 



However, for sharpness of detail, nothing equals the perfect vacuum 

 tube, having its electrodes one mm. apart. Such a tube has been de- 

 signed by one of us, but we have not been able to get the proper 

 exhaustion. 



As to other sources of Rontgen rays, we have tried a torrent of elec- 

 tric sparks in air, from a large battery, and have obtained none. Of 

 course, coins laid on or near the plate under these circumstances, pro- 

 duce impressions, but these are, of course, induction phenomena. 



As to sunlight, Tyndall, Abney, Graham Bell and others, have 



