358 POPULAR SCIENCE MONTHLY. 



spark, the brass balls (in line) and the rods that support them, and the 

 sound wave, which appears in the simplest case as a circle of light and 

 shade surrounding the balls. By placing an obstacle in the way of the 

 wave we get the reflected wave or echo, and we shall see that the form 

 of this echo may be very complicated. 



It will be well at the outset to remind the reader of the close 

 analogy between sound and light. A burning candle gives out spher- 

 ical light waves, just as the snapping sparks give out sound waves. 

 The ~form of the reflected light wave will be identical with that of a 

 sound wave reflected under similar conditions. As we can not see the 

 light waves themselves, we can only determine their form by calculation, 

 and it is interesting to see that the forms photographed are identical 

 in every case with the calculated ones. The object in view was to 

 secure acoustical illustrations of as many of the phenomena connected 

 with light as possible. We will begin with the very simplest case of all: 

 the reflection of a spherical sound wave from a flat surface, correspond- 

 ing to the reflection of light from a plane mirror. It can be shown 

 by geometry that the reflected wave or echo will be a portion of a 



Fi<.. i. Sound Wave Reflected from a Plane Surface. 



sphere, the center of which lies as far below the reflecting surface as 

 the point at which the sound originates is above it. In the case of light, 

 this point constitutes the image in the mirror. Eeferring to the photo- 

 graph, we see the reflected wave in three successive positions, the in- 

 terval between the sound spark and the illuminating spark having been 

 progressively increased. The brass balls are shown at A, and beneath 

 them the flat plate B, which acts as a reflector. In the first picture the 

 sound wave C appears as a circle of light and shade, and has just 

 intersected the plate. The echo appears at D. In the next two pic- 

 tures the original wave has passed out of the field, and there remains 

 only the echo. 



It may, perhaps, be not out of place to remind the reader of the 

 relation between rays of light and the wave surface. What we term 

 light rays have no real existence, the ray being merely the path 

 traversed by a small portion of consecutive wave surfaces. Since the 

 wave surface always moves in a direction perpendicular to itself, the 

 rays are always normal to it. For instance, in the above case of a 

 spherical wave diverging from a point, the rays radiate in all directions 



