Intelligence and Miscellaneous Articles. 



383 



to diminish the size of the source and increase its intensity, have 

 the fault that, owing to the great heat developed, the glass is very 

 apt to crack. I have had some success with a tube which 1 made 

 in which the kathode hangs as a pendulum from the centre of a 

 spherical bulb, by the slow rotation of which one brings a fresh 

 and cold surface into the focus continually, thereby avoiding over- 



heating. The concave kathode hangs by an aluminium wire from a 

 short cylinder of aluminium fastened into a glass tube, through 

 w T hich a platinum wire passes which lies in the axis of rotation. 

 The anode is also in the axis of rotation, so that the connexions 

 with the coil can be easily made. My tube has the fault that 

 many kathode rays emanate from the upper surface of the concave 

 plate and are lost. This might be overcome by covering the kathode 

 with a cap of glass. As a suggestion for further experimenting, 

 this note may be of interest to persons working with the new 

 rays. 



Berlin, March 8, 1896. 



NOTE ON ELEMENTARY TEACHING CONCERNING 

 FOCAL LENGTHS. 



To the Editors of the Philosophical Magazine. 



GrENTLEMEN, 



"With respect to Prof. Lodge's question on page 152 as to the 

 simplest convention of signs in dealing with focal lengths for junior 

 students, it appears to me to be a matter which can be best settled 

 by one who has had experience in teaching two or more methods. 

 Hence, as this is a qualification to which I can lay no claim, I 

 forbear to dogmatize. 



It would seem to me, however, undesirable, apart from weighty 

 reason, to teach an elementary student to let the algebraical sign 

 of a given line depend, not upon its direction, but upon the physical 



