182 



Scientific Proceedings, Royal Dublin Society. 



hence the sensitiveness increases directly in proportion to the square of the 

 linear dimensions of the plates and inversely as the square of their distance 

 apart. The negative sign, of course, indicates that as the plates are moved 

 apart the grid potential v diminishes. 



In the next apparatus set up, two circular metal plates 7 cms. in diameter 

 were utilized. One of these was cemented to the table of a Leitz microscope. 

 The fine adjustment-screw of this had a large divided head, each division of 



50 



Kr- 



10^ cm. 



52 



Fig. 6. 



.54 



which corresponded to a vertical movement of the microscope tube through 

 ^p'pp cm., and 3- of a division was easily readable. The strong spring of the fine 

 adjustment effectually prevented backlasli. A strong glass tube was supported 

 in the microscope tube by two stout plugs which held it very rigidly, and the 

 lower extremity of this tube was introduced into a small metal cup attached 

 to the centre of the upper condenser-plate. While tliis plate rested on its 



