August 9, 1900] 



NATURE 



343 



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. A good-sized test tube half full of mercury stand- 

 ing in a jar of mercury is the easiest arrangement to fit 

 up. This limits the length and brilliancy of the illumin- 

 ating-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 improvement resulted. With 

 this form of illuminator I found that five or six times as 

 much light could be obtained as by the old method of 

 passing the spark between two brass balls. 



The spark is flattened out into a band, and is kept 

 always in the same plane, the light issuing in a thm 

 sheet from between the plates. By this arrangement we 

 secure a light source of considerable length, great in- 

 tensity, and bounded by straight edges, the three essentials 

 for securing good results. The glass plates, with the 

 ribbon terminals between them, must be clamped in some 

 sort of a holder and directed so that the thin sheet of 

 Light strikes the lens : this can be accomplished by 

 darkening the room, fastening a sheet of paper in front 

 of the lens, and then adjusting the plates so that the 



In the first series the pictures were so small that it was 

 necessary to enlarge them several diameters. Those of 

 the new series, owing to the use of an eight-inch mirror 

 in place of the five-inch lens, and an objective of larger 

 aperture and longer focus, required no enlarging. 



The Wave- Front Photographs. 

 In the study of optics we may treat the subject of 

 regular reflection in two ways, by rays and by wave- 



paper is illuminated as much as possible. The image 

 formed by the lens will be found to have very sharp 

 straight edges,' on one of which the edge of the 

 diaphragm can be set in such a manner as to allow 

 but very little light to pass when the intervening medium 

 is homogeneous ; a very slight change, however, in any 

 portion may be sufficient to cause the entire amount of 

 light passing through that portion to pass below the 

 diaphragm and enter the telescope. 



The photographs were made by substituting a photo- 

 graphic objective for the telescope, in the focal plane of 

 which a vertical board was mounted to support the plate. 

 The room was darkened, a plate held in position, and a 

 single spark made to pass between the knobs by pulling 

 a string connected with the hammer of the induction 

 coil. The plate was then moved a trifle and a second 

 impression secured in the same way. This obviated 

 several of the difficulties experienced in the earlier work. 

 The images never overlapped, and the hot air from the 

 spark did not appear in the pictures. About thirty-five 

 images were obtained on each plate in less than a 

 minute, from which it was usually possible to pick a 

 series showing the wave in all stages of its development, 

 owing to the variations in the time-interval between the 

 two sparks. 



' If more than one image appears it means that the plane of the glass 

 ites of the illuminator does not lie p:trallel to the optical axis of the 

 \^tem. It is of prime importance to secure a single image. 



fronts. When spherical waves of light are reflected from 

 a plane surface, we know that the reflected waves are 

 also spherical in form, the centre of curvature being a 

 point just as far beneath the reflecting surface as the 

 source of light is above it. In the first of the series of 

 photographs we have the reflection of a spherical wave 

 of sound by a flat plate of glass, the wave appearing as a 

 circle of light and shade surrounding the image of the 

 balls between which the spark passed 

 (Fig. 3). The reflected wave or echo 

 from the plate is seen to be spherical, 

 with a curvature similar to the incident 

 wave. 



When we have a source of light in 

 the focus of a parabolic mirror, the rays 

 leave the mirror's surface parallel to one 

 another, and move out in an intense 

 narrow beam. Treating this case from 

 the wave-front point of view, we ascer- 

 tain by the usual geometrical construc- 

 tion that the spherical wave is changed 

 by reflection into a plane or flat wave 

 which moves out of the mirror without 

 further divergence. In the picture 

 (Fig. 4), only a portion of the parabolic 

 reflector is shown near the bottom. 

 The sound-wave starts in the focus, and the reflected 

 portion appears quite flat.^ 



What happens now if we use a spherical mirror in the 

 same way .? 



Owing to the spherical aberration the reflected rays 

 are not strictly parallel, or the reflected wave is not a true 

 plane. Let us start a sound-wave in the focus of such a 



NO. 1606, VOL. 62] 



mirror, and follow the reflected portion out of the mirror 

 (Fig. 5). We notice that near the axis of the mirror 

 the effect is much the same as in the case of the para- 

 bola, that is, the reflected front is plane. Thus we are 



1 In this series and some others left and right have been inadvertently 

 interchanged by the engraver. The series should be followed by the 

 numbers. 



