of Simultaneous Contrast- Colour, 163 



The grey ring was placed on the emerald-green ground near 

 a Holtz machine, which gave a very bright flash of 8 centim. 

 long. The condensing surface on the two jars connected with 

 the electrodes of this machine equalled 135 square centi- 

 metres. Professor 0. N". Rood* measured the duration of the 

 flash of Leyden jars of 738 and 71 square centimetres of 

 surface, charged by an inductorium. The durations of the 

 discharges of these jars were respectively * second and 

 n „ nn \ nnn of a second. From these measures I infer that 



zo>000.000 



the duration of the flash of the Holtz machine did not exceed 

 the * of a second. With short striking distances 



between the electrodes the flash is formed of several separated 

 acts, as shown by Henry f, Feddersent, Eood§, and Mayer ||. 

 In the case of the discharge of a large inductorium the 

 writer (||) has shown that when the striking distance between 

 brass ball electrodes is only one millimetre, with a Leyden jar 

 of 242 square centim. of surface in the circuit, the discharge 

 lasts ^ ? of a second and is formed of over 120 separate sparks; 

 but as the striking distance is increased the discharge is 

 formed of fewer and fewer components, till at a striking dis- 

 tance between 1 and 2 centim. the discharge is reduced to a 

 single flash. In the following experiments the striking dis- 

 tance is 8 centim., and a single flash was given, whose 

 duration we may safely assume was less than . nn \ nnn of a 



J J 1,000.000 



second. 



In a dark room, at night, the flash of this machine gave 

 vivid contrast-colours, — the grey ring appearing bright pink 

 on an emerald-green ground, and of a bright yellow on an 

 ultramarine ground. The after-images of these effects lasted 

 about ^ of a second. 



A rod was placed in front of a white cardboard screen and 

 the shadow of the rod was formed on the screen by a candle. 

 The distance of the candle was such that the white cardboard 

 appeared equally brilliant when illuminated only by the candle, 

 or, only by the electric flash. At the moment of the flash the 

 appearance presented was very remarkable. From the shadow 

 of the candle appeared suddenly to shoot a dark screen, which 

 had superposed the shadow of the candle 3 and which shot to 

 the side of the shadow and appeared of a bright golden 

 orange, while the apparently uncovered shadow of the candle 

 appeared of a brilliant cobalt-blue ; to my eye exactly as 

 though an opaque screen had been suddenly removed from a 



* Amer. Journ. Sci. Sept. 1871. + Proc. Amer. Phil. Soc. 1842. 



X Pogg. Ann. vol. cxvi. p. 132. § Amer. Journ. Sci. Oct. 1872. 



II Amer. Journ. Sci. Dec. 1874. 



