3 i8 THE POPULAR SCIENCE MONTHLY, 



than the millionth or the half a millionth of a second, but how much 

 less time it might occupy remained to be determined. 



Prof. Rood now prepared for a more rigorous course of experi- 

 ments. He used a small Leyden jar, with a surface of eleven inches, 

 about equal to a moderate-sized wine-glass. To secure greater exact- 

 ness of observation, he devised a peculiar micrometer, consisting of 

 five lines ruled on a plate of glass smoked by lamp-black. This plate 

 was placed between I and S {see Fig. 4), but quite near to the latter, 

 and an image of the lines reflected from the mirror was formed on the 

 clear glass at i. The lines were observed by a microscope magnifying 

 ten diameters. In using this micrometer, the measurement was effected 

 by noticing at what rate of the revolving mirror the lines in the image 

 at i were obliterated, this obliteration being due to the circumstance 

 that by the motion of the mirror the dark lines were superposed on 

 the bright lines. The individual spark now produced was about a 

 millionth of a second in duration, but the faint train was still observ- 

 able. There was still the brilliant body of the spark appearing, first 

 followed by a faint streak of less than one-hundredth the illuminating 

 power of the first stage. The diagram, Fig. 6, represents the intensity 



Fig. 6. 



Duration. 



and time of the spark. The elevation, or peak, a b, shows the intensity 

 of the first compact body of the spark, and the line a c the duration 

 of the whole effect. The point was to get the time of a b, which 

 Prof. Rood had proved must be regarded as a distinct act in the suc- 

 cession of effects. All precautions for observation being carefully 

 made, the driving- weight was gradually increased, and the speed of 

 the mirror carried up to 350 revolutions per second, when the lines of 

 the image, which at first remained visibly as distinct as with a sta- 

 tionary mirror, became regularly less distinct, and at length vanished 

 by the gradual superposition of the white and black lines. Prof. Rood 

 says: "It was proved successively that the duration was less than 

 eighty, sixty-eight, fifty-nine, fifty-five billionths of a second ; and, 

 finally, the lines, after growing fainter and fainter, entirely disap- 

 peared, giving as the result a duration of forty-eight billionths of a 

 second." By reducing the striking distance, a still lower figure was 



