262 The Bight Eon. Lord Bayleigh [Feb. 6, 



powder into the flame of a spirit lamp, and the flash will be produced. 

 Please give your attention to the disc, for the question is whether 

 the present uniform grey will be displaced by a perception of the 

 individual black and white sectors. [Experiment.] You see the flash 

 was not instantaneous enough to resolve the grey into its components. 



I want now to contrast with that mode of illumination one 

 obtained by means of an electric spark. We have here an arrange- 

 ment by which we can charge Leyden jars from a Wimshurst 

 machine. When the charge is sufficient, a spark will pass inside a 

 lantern, and the light proceeding from it will be condensed and 

 thrown upon the same revolving disc as before. The test will be 

 very much more severe ; but severe as it is, I think we shall find that 

 the electric flash will bear it. The teeth on the outside of the disc 

 are very numerous, and we will make them revolve as fast as we can, 

 but we shall find that under the electric light they will appear to be 

 absolutely stationary. [Experiment.] You will agree that the 

 outlines of the black and white sectors are seen perfectly sharp. 



Now, by means of this arrangement we might investigate a limit 

 to the duration of the spark, because with a little care we could 

 determine how fast the teeth are travelling — what space they pass 

 through in a second of time. For this purpose it would not be safe 

 to calculate from the multiplying gear on the assumption of no slip. 

 A better way would be to direct a current of air upon the teeth them- 

 selves, and make them give rise to a musical note, as in the so-called 

 siren. From the appearance of the disc under the spark we might 

 safely say, I think, that the duration of the light is less than a tenth 

 of the time occupied by a single tooth in passing. But the spark is 

 in reality much more instantaneous than can be proved by the means 

 at present at our command. In order to determine its dui*ation, 

 it would be necessary to have recourse to that powerful weapon 

 the revolving mirror ; and I do not, therefore, propose to go fui'ther 

 into the matter to-night. 



Experiments of this kind were made some twenty years ago by 

 Prof. Rood, of New York, both on the duration of the discharge of a 

 Leyden jar, and also on that of lightning. Prof. Rood found that the 

 result depended somewhat upon the circumstances of the case ; the 

 discharge of a small jar being generally more instantaneous than that 

 of a larger one. He proved that in certain cases the duration of the 

 principal part of the light was as low as one twenty-five-millionth 

 part of a second of time. That is a statement which probably con- 

 veys very little of its real meaning. A million seconds is about 

 twelve days and nights. Twenty-five million seconds is nearly a 

 year. So that the time occupied by the spark in Prof Rood's experi- 

 ment is about the same fraction of one second that one second is of a 

 year. In many other cases the duration was somewhat greater ; but 

 in all his experiments it was well under the one-millionth part of a 

 second. In certain cases you may have multiple sparks. I do not 

 refer to the oscillating discharges of which Prof. Lodge gave us so 



