Hutchins and Robinson — Concerning Crookes Tubes. 465 



is best. It is very difficult to have other conditions sufficiently 

 uniform to enable one to judge, where differences are small. 



We have made the anode in the form of a wire of aluminum, 

 a flattened strip of it, and, as stated above, in the form of a 

 platinum reflector. As yet we have got our best results from 

 the platinum. One rather interesting result obtained was, 

 that when the anode was in the form of an aluminum disc 

 parallel to the cathode and nearly large enough to close the 

 tube, it gave little or no interference with the X-ray. We 

 made one on a hinge so that it could be swung out of the path 

 of the ray or in at pleasure, and the effect on the photographic 

 plate was the same in either position. 



Source of the rays. — Being able to construct tubes of any 

 form, we have made many experiments as to the source of the 

 rays, whether from the cathode or anode. One was in this 

 way : — Two tubes were joined together parallel so that they 

 were exhausted together. The cathode rays could be made to 

 pass down one tube and the anode rays (if such existed) down 

 the other, and either screened off at will. We found that the 

 anode rays affected the plate but slightly, and that practically 

 all the effect came from the cathode. 



Intensity of effect. — We do not intend to convey the impres- 

 sion that these home-made tubes we have described are simply 

 good enough for experiment and valuable from their cheap- 

 ness. We believe also that they are more effective than 

 others. We have made good negatives of bones of the hand, 

 arm, including the elbow, foot, ankle, etc., all with remarkably 

 short exposures ; have taken impressions perfectly distinct 

 through nine inches of wood in less than five minutes; have 

 taken perfectly the bones of the hand through thin sheet 

 zinc in two minutes and through the slide of the plate holder 

 in five seconds. The ordinary coin and key impression require 

 not over one or two seconds with our best tubes. 



Remarks upon pumping. — The interest in the subject at 

 present may make some remarks upon pumping here in place, 

 most of all, since many have found great difficulty in this 

 respect. 



It is here supposed that the pump has a three-way cock 

 above its bulb, opening in its two positions between the bulb 

 and fork and the bulb and outer air ; and that above this three- 

 way cock are one or two cocks of the ordinary kind. Let the 

 three-way cock be called A, the others B and C in order. 

 Let the position in which A puts the bulb in communication 

 with the fork be position 1 ; and that in which it puts the 

 bulb in connection with B, C, and the outer air, position 2. 

 The ordinary process of pumping with the use of A alone is 

 supposed to be understood. After a greater or less number of 



