452 Dr. E. Goldstein on the 



If the reflection were similar to that of a mirror, the form and 

 magnitude of the surface F would change perceptibly. This 

 surface, in all the experiments so far described, and in this one 

 also, resembles an ellipse of small excentricity, where D has 

 the position first mentioned. Its smaller axis, which falls in 

 the plane of r r i} is about twice as large as the diameter of r x 

 in the vessel C, which is about 3 centim. in diameter. If now 

 we mark the position, form, and magnitude of F on the out- 

 side of the tube C when the diaphragm presents its edge only 

 to the kathode-rays, and then place the diaphragm at right 

 angles to the rays, we find that the position, form, and magni- 

 tude of F remain unchanged', only its luminosity is now consi- 

 derably diminished. 



If, instead of the diaphragm with a slit, a plate without 

 openings is introduced into the chamber B capable of free 

 motion, so as to cut off at pleasure either the upper or lower 

 half (and also the right-hand or the left-hand half of the 

 kathode-pencil) by covering the corresponding portion of the 

 mouth of r, then also the position, form, and magnitude of the 

 surface F remain unchanged; the luminosity only of the whole 

 surface decreases, but most in the half which is opposite to the 

 half that has been intercepted. Thus, for example, the surface 

 is darkest in the upper half when it is the lower half of the 

 kathode-pencil which is intercepted. 



We easily see how these observations, inconsistent with 

 optical reflection, entirely agree with the assumption of a dif- 

 fuse reflection of the kathode-rays. 



Tubes of the form of fig. 5 are better adapted for the further 

 study of this diffuse reflection than the vessels employed by 

 Wiedemann. The rays emitted b}' the kathode k which pass 

 through the connecting tube r into the wider cylinder Z fall 

 then upon the plate P, which is fastened to a wire d insulated 

 with glass inside Z. The cylinder is closed air-tight by the 

 caoutchouc stopper k, by removing which the plate P can be 

 exchanged for another; or other changes in the apparatus can 

 be made. 



If the plate P consists of phosphorescent glass, then the 

 rays which fall upon it directly produce at the plate s simply 

 an oval very bright green phosphorescent surface. We see, 

 however, distinctly how the diffuse reflection from this surface 

 causes the whole wall of the tube Z lying above the plane of 

 P up to the stopper k to phosphoresce with subdued green 

 light, which is weaker the further the portion of the wall is 

 removed from s. 



If the plate P be covered with chalk, its surface at s shines 

 with orange-red light, but the wall of Z presents a green 



