220 



Mr. R. W. Wood on Photography of 



absolute even feeble illumination, but a good achromatic 

 telescope-objective is perfect enough for every purpose. A 

 more complete discussion of the operation of the apparatus 

 will be found in Toepler's original paper in the Annalen. 

 The sound-waves, which are regions of condensation, and 

 consequent greater optical density, make themselves apparent 

 in the same w;iy as the globular mass of air already referred 

 to. They must be illuminated by a flash of exceedingly 

 short duration, which must occur while the wave is in the 

 field of view. 



Toepler showed that this could be done by starting the sound- 

 wave with an electric spark, and illuminating it with the 

 flash of a second spark occurring a moment later, while the 

 wave was still in the field. A diagram of the apparatus used 

 is shown in fig;. 2. In front of the lens are two brass balls 

 (a, a), between which the spark of an induction-coil passes, 



Fig. 2. 



Illum/nato* 



immediately charging the leyden-jar c, which discharges 

 across the gap at e an instant later. The capacity of the jar 

 is so regulated that the interval between the two sparks is 

 about one ten-thousandth of a second. The field of the lens 

 is thus illuminated by the flash of the second spark before the 

 sound-wave started by the first spark has gone beyond the 

 edge of the lens. 



To secure the proper time-interval between the two sparks 

 it is necessary that the capacity of the jar be quite small. This 

 limits the length and brilliancy of the illuminating-spark, and 

 with the device employed by Toepler I was unable to get 

 enough light to secure photographs of the waves. After 

 some experimenting I found that if the spark of the jar was 

 passed between two thin pieces of magnesium ribbon pressed 

 between two pieces of thick plate-glass, a very marked im- 

 provement resulted. With this form of illuminator I found 



