250 



NATURE 



[July i6, 1891 



only a very short time. A good deal of use has been made 

 in this way of what is known as the magnesium flash 

 light. A cloud of magnesium powder is ignited, and 

 blazes up quickly with a bright light of very short dura- 

 tion. Now I want to compare that mode of illumination 

 with another, in order to be able to judge of the relative 

 degree of instantaneity, if 1 may use such an expression. 

 We will illumine for a short time a revolving disk, com- 

 posed of black and white sectors ; and the result will 

 depend upon how quick the motion is as compared with 

 the duration of the light. If the light could be truly 

 instantaneous, it would of necessity show the disk ap- 

 parently stationary. I believe that the duration of this 

 light is variously estimated at from one-tenth to one- 

 fiftieth of a second ; and as the arrangement that I have 

 here is one of the slowest, we may assume that the time oc- 

 cupied will be about a tenth of a second. I will say the words 

 one, two, three, and at the word three Mr. Gordon will pro- 

 ject the powder into the flame ofaspirit lamp, and the flash 

 will be produced. Please give your attention to the disk, 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 arrangement 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 disk 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 disk 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 out- 

 lines of the black and white sectors are seen perfectly 

 sharp. 



Now, by means of this arrangement we might investi- 

 gate a limit to the duration of the spark, because with a 

 little care we could determine hov/ fast the teeth are tra- 

 velling—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 themselves, and make them give rise to a musical 

 note, as in the so-called siren. From the appearance of 

 the disk 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 duration, it would be necessary to have re- 

 course to that powerful weapon the revolving mirror ; and 

 I do not, therefore, propose to go further 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 light- 

 ning. Prof. Rood found that the result depended some- 

 what 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 conveys 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 

 experiment is about the same fraction of one second that 

 one second is of a year. In many other cases the dura- 

 tion was somewhat greater ; but in all his experiments 



NO. 1 133, VOL. 44] 



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 interesting an account last year ; Prof. Rood's 

 multiple discharge was not of that character. It con- 

 sisted of several detached overflows of his Leyden jar 

 when charged by the Rhumkorff coil. One number 

 mentioned for the total duration was one six-thousandth 

 part of a second ; but the individual discharges had the 

 degree of instantaneity of which I have spoken. 



It is not a difficult matter to adapt the electrical spark 

 to instantaneous photography. We will put the lantern 

 into its proper position, excite the electric sparks within 

 it, causing them to be condensed by the condenser of the 

 lantern on to the photographic lens. We will then put the 

 object in front of the lantern-condenser, remove the cap 

 from the lens, expose the plate to the spark when it 

 comes, and thus obtain an instantaneous view of whatever 

 may be going on. I propose to go through the opera- 

 tion of taking such a photograph presently. I will not 

 attempt any of the more difficult things of which I shall 

 speak, but will take a comparatively easy subject — a 

 stream of bubbles of gas passing up through a liquid. 

 In order that you may see what this looks like when 

 observed in the ordinary way, we have arranged it here 

 for projection upon the screen. [Experiment.] The gas 

 issues from the nozzle, and comes up in a stream, but so 

 fast that you cannot fairly see the bubbles. If, however, 

 we take an instantaneous picture, we shall find that the 

 stream is decomposed into its constituent parts. We 

 arrange the trough of liquid in front of the lantern which 

 contains the spark-making apparatus^ — [Experiment] — 

 and we will expose a plate, though I hardly expect a good 

 result in a lecture. A photographer's lamp provides 

 some yellow light to enable us to see when other light 

 is excluded. There goes the spark ; the plate is exposed, 

 and the thing is done. We will develop the plate, and 

 see what it is good for ; and if it turns out fit to show, 

 we will have it on the screen within the hour. 



In the meantime, we will project on the screen some 

 slides taken in the same way and with the same subject. 

 [Photograph shown.] That is an instantaneous photo- 

 graph of a stream of bubbles. You see that the bubbles 

 form at the nozzle from the very first moment, contrasting 

 in that respect with the behaviour of jets of water pro- 

 jected into air (Fig. i). 



The latter is our next subject. This is the reservoir 

 from which the water is supplied. It issues from a nozzle 

 of drawn-out glass, and at the moment of issue it consists 

 of a cylindrical body of water. The cylindrical forrn is 

 unstable, however, and the water rapidly breaks up into 

 drops, which succeed one another so rapidly that they 

 can hardly be detected by ordinary vision. But by 



