1 84 SCIENCE PROGRESS. 



magnet in various aspects with respect to the line of dis- 

 charge or of conduction between two plates in vacuo, such 

 as are mentioned just above. 



They found that if the lines of magnetic force agreed in 

 direction with the lines of electrostatic force they had no 

 appreciable influence on the discharging power of light ; 

 but that as soon as the magnetic and the electrostatic 

 lines crossed, the discharging power of light was definitely 

 diminished and interfered with. 



QUESTION OF INTENSITY OR TOTALITY OF 

 ILLUMINATION. 



Experiments made in my laboratory were directed to 

 ascertain whether the action of light depended on its con- 

 centration or on its total amount. To this end a large 

 metal plate was set at different distances from the source, 

 in a cone of rays, so that at one time a very small area, and 

 at another time a larger area, was exposed to the same 

 total quantity of light. 



No very minutely accurate measurement was possible, 

 because an arc-light is subject to capricious variations, — the 

 long fizzing arc being much the most effective; but certainly 

 if there is any effect due to concentration it is very small. 

 On the average the rate of leak was just the same when 

 the light was spread over a large area as over a small. 



INFLUENCE OF THE DIRECTION OF THE LUMINOUS 



VIBRATION. 



Another vital discovery was made by the Wolfenbiittel 

 experimenters, by aid of their beautiful method of em- 

 ploying the alkali metals for the purpose in partial 

 vacua in glass bulbs. The alkali metals are generally 

 superior to others by reason of the fact that ordinary visible 

 light is powerfully effective in their case, so that glass is no 

 deterrent, and candles or diffuse daylight may be used. 

 This is convenient for measuring purposes, because of their 

 great steadiness as compared with an arc ; and another con- 

 venience is the possibility of using a liquid metallic surface, 

 viz., that of the liquid potassium-sodium alloy. Mercury is 

 comparatively inert, but a liquid surface has the great merit 



