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Page:Popular Science Monthly Volume 57.djvu/371

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See the third and fourth pictures of the series. The bowl of the crater shrinks to a point half-way between the surface of the mirror and its center of curvature, and represents that portion of the light which comes to a focus, while the sides of the cone run in under the collapsing bowl, and eventually cross. (No. 6 of the series.) From now on the portion which has come to a focus diverges, uniting with the sides of the cone, the whole passing out of the mirror in the form of a horseshoe.

We will now consider a case of refraction, and show the slower

PSM V57 D371 A wave from an elliptical mirror.png
Fig. 5. A Wave from an Elliptical Mirror.
PSM V57 D371 A wave starting from outside of the field of lens.png
Fig. 6. A Wave starting Outside the Field of the Lens.
PSM V57 D371 A case of refraction.png
Fig. 7. A Case of Refraction.

velocity of the sound wave in carbonic acid. A narrow glass tank, covered with an exceedingly thin film of collodion, was filled with the heavy gas and placed under the brass balls. When the sound wave strikes the collodion surface, it breaks up into two components, one reflected back into the air, the other transmitted down through the carbonic acid. An examination of the series shows that the reflected wave in air has moved farther from the collodion film than the transmitted wave, which, as a matter of fact, has been flattened out into