Page:Popular Science Monthly Volume 36.djvu/58

48 grating is made is beyond the scope of the present paper. Assuming its use, the sensitive flame enables us to detect a focal area of noise, at which the flame is violently agitated, and around this are alternate rings of silence and fainter noise diminishing in strength with increase of distance from the central focus.

By admitting light through two small openings close together, the waves coming from a distant bright point and hence reaching the two openings in the same phase, hyperbolic lines of interference like those shown in Fig. 8 were traced in space by Fresnel. The writer has recently done the same with sound-waves, using the sensitive flame as an explorer. Bands of alternate noise and silence have in like manner been traced by him in air, produced by interference between the waves proceeding directly from the whistle and those reflected from a smooth surface placed horizontally on the table.

The wave theory of sound has long been impregnable; but these beautiful analogies between light and sound, though provided for by theory, have been experimentally demonstrated only recently. Such new and unexpected confirmations, new points of contact, are always welcome, even though they be not needed for the establishment of a theory. They are the results of prevision based on the assumption that an elastic material medium is needed for the propagation of sound, and are wholly inexplicable on any theory of emanation analogous to Newton's emission theory of light.



ODERN biology has made familiar the idea that animals are not fixed and unalterable in their bodily structure and functions, but, within a certain range, respond by changes in themselves to changes in their physical surroundings. This has always been observed to be true for the individual animal, as in the changes undergone in adaptation to the seasons of the year; mammals, for instance, acquiring a thicker coat of hair at the approach of winter, and reptiles and other classes passing into a low state of functional activity called hibernation or winter sleep. But it has now been well shown that this principle of modification by environment applies to species as well as to individuals. That