284 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



spectrum placed on the edges of the plate — this by the use of the slits 

 shown in Figure B ; then the first operation was repeated, and the arc 

 spectrum again thrown upon the centre. But in most cases, always 

 during the latter part of the investigation, the shutter was so arranged 

 that the vertical superposition of the first and third exposures was not 

 perfect, so that the central portion of the plate containing the arc 

 spectrum presents some such appearance as that shown in Figure C. A 

 mechanical shift may thus be easily detected. 



Production of the Sources of Light. 



The spark was produced in the following manner: The city current 

 (110 volts, 133 cycles per second) was led into a 5 kilowatt transformer 

 (110 to 15,000, 30,000 or 60,000 volts) manufactured by the Central 

 Laboratory Supply Co., of Lafayette, Indiana, in series with which 

 was a home-made water rheostat. The secondary voltage ranged from 

 approximately 1600 to 14,000 volts, as the ratio of transformation 

 which was employed was 30,000 to 110. The high potential current was 

 led into glass-paraffin-tin condensers giving a range of capacity varying 

 from about 0.008 to 0.08 microfarads (see Table I). Both the charging 

 and discharging high potential wires were very heavy (about No. 8 

 standard wire gauge). 



TABLE I. 

 Capacity of Condensers. 



The self-induction coil employed consisted of three, concentric cylindrical 

 coils of dimensions given below (see Table II). Generally, when 

 photographing the spark, there was placed in the rear of it, at a distance 

 i \ 80 cm., a 12-inch Westinghouse fan, the strong air current from which 

 was directed upon the spark-gap by a megaphone 70 cm. long, the smaller 



