Intelligence and Miscellaneous Articles. 317 



surface of the mirror from the effects of the spark, a thin plate of 

 quartz was placed iu front of it. It was found that the copper 

 mirror showed no limit of selective absorption by reflexion for 

 wave-lengths of light produced b} r burning copper at the limits of 

 the copper-spectrum, that is, at wave-length 2100. The photo- 

 graphic plate taken by this method showed all the lines that the 

 plates showed which were taken by the direct light of the spark 

 unreflected and unabsorbed by any medium. The palladium mirror 

 was substituted for the copper mirror, and also showed no limit of 

 selective absorption above wave-length 2100. We are led to con- 

 clude, therefore, that the metallic surface of the speculum metal 

 upon which the lines are ruled which form the diffraction-grating 

 does not fix by selective absorption the limit of metallic spectra at 

 1800 to 2100. This limit more likely resides in the materials 

 forming the sensitive emulsion with which the sensitive plates are 

 coated. We have found that a marked difference exists in different 

 emulsions in regard to sensitiveness to ultra-violet light. The 

 various staining processes, which enhance to such a marked degree 

 the sensitiveness of photographic plates to wave-lengths of greater 

 length, do not seem to affect the limit of metallic spectra in the 

 ultra-violet. Thus, plates stained with erythrosine, which are 

 extremely sensitive to yellow 7 and green light, continue to give the 

 same limit in the ultra-violet after staining as they did before they 

 were submitted to the staining process. 

 Jefferson Physical Laboratory, 

 Cambridge, U.S. 



ON A KIND OF ELECTRICAL CURRENT PRODUCED BY ULTRA- 

 VIOLET RAYS. BY M. A. STOLETOW. 



The researches of Hertz, E. Wiedemann and Ebert,and Hallwachs 

 having demonstrated the influence of the ultra-violet rays on elec- 

 trical discharges of high potential, I conceived the idea of trying if 

 a similar effect could be obtained with electricity of low potential. 



Two metal disks, 22 centim. in diameter, are placed vertically 

 opposite each other in front of a lantern with a voltaic arc fed 

 by a dynamo (12 amperes, 70 volts). The disk turned towards the 

 lamp is of wire gauze, the other is solid ; the two form a kind of 

 condenser, of which one armature may be illuminated on its internal 

 face through the meshes of the other. 



I take any battery, and connect its negative pole to the solid 

 disk and the gauze disk is connected with the positive pole. A 

 Thomson's astatic galvanometer is introduced into the circuit, 

 which is broken by an air-space. 



When the lamp is lighted the galvanometer is deflected and 

 remains so ; a kind of current traverses the circuit. Any opaque 

 screen, or any kind of glass, placed between the arc and the gauze 

 cause the deflections to disappear ; a quartz plate only enfeebles it 

 to a slight extent. The deflection is constant as long as the lamp 

 acts ; any irregularity is at once apparent in the changes of current. 



