464 Hutchins and Robinson— Concerning Croohes Tubes. 



the tube itself, then a bit of softer glass, and upon that very 

 soft lead glass for the seal. 



Shape of the tube. — A good tube should throw shadows as 

 sharp as possible and develop the rays as powerfully as possible. 

 It should easily appear that the ordinary spherical form meets 

 neither of these conditions. 



To produce a sharp shadow the radiant must be small. It 

 was found that a picture could be taken upon any side of a 

 spherical bulb ; making it probable at least that the entire sur- 

 face is a source of radiation. 



In the matter of strong action also, the spherical form is 

 inferior. This is for two reasons : — first, glass more or less 

 extinguishes the rays according to its thickness, therefore, the 

 larger the bulb the more opaque it must be, for it must be 

 thick enough to stand the atmospheric pressure. 



Secondly : there is a comparatively large amount of radiant 

 or conducting matter within the spherical bulb which diffuses 

 the energy of the discharge. 



Proof of the second point was obtained as follows. A moder- 

 ately thick bulb about three inches in diameter was blown, 

 and upon this a spot one inch across was blown out very thin, 

 forming a smaller hemispherical bulb upon the first. Opposite 

 this thin window was the concave cathode. This bulb proved 

 better than the ordinary sort, but far inferior to tubes about to 

 be described. A second experiment was made with a tube 

 blown thin along one side for a space of three inches, and 

 opposite this was the cathode in the form of a quarter cylinder. 

 The performance of this was also inferior. 



Without going into the details of many similar experiments, 

 it will be sufficient to say that we have found that a simple 

 straight tube from one-half to one inch in diameter, having a 

 small and very thin bulb for a cathode window, has given the 

 most satisfactory results. In length it may be from four to 

 eight inches. The bulb may be blown at the bottom of the 

 tube, the cathode placed at the top, and the anode across the tube 

 just above the bulb. Better results are, however, produced by 

 using a bit of platinum foil for an anode, inclining it about 

 forty-five degrees to the cathode ray. In this case the small 

 bulb may simply be blown out upon the side of the tube and 

 the electrodes put in at the two ends, so that the cathode ray 

 will be reflected into the bulb. 



/Shape and disposition of the electrodes. — We have made the 

 cathode in the form of a wire, a flat plate, a convex plate and a 

 concave plate. The concave form proves the best in every 

 case. We have made it of varying size, up to an inch or more 

 in diameter, and have not come to any conclusion as to which 



