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POPULAR SCIENCE MONTHLY. 



a broad strip of metal bent into a circle.* Intricate as these wave sur- 

 faces are, they have all been verified by geometrical constructions, as I 

 shall presently show. 



Another very interesting case of reflection is that occurring inside 

 an elliptical mirror. When light diverges from one of the two foci 

 of such a mirror, all the rays are brought accurately to the other focus. 

 If rays of light come to a focus from all directions, it is evident that 

 the wave surface must be a sphere, which, instead of expanding, is 

 collapsing. This is very beautifully shown in the photographs. The 

 sound wave starts in one focus and the reflected wave, of spherical form 

 also, shrinks to a point at the other focus. (See fig. 5.) 



In the next series the wave starts outside of the field of the lens, 



Fig. 3. A Wave Reeeei tei> from 



>f a Spheric. 



! . 3 I 



m^ 



Fig. 4. A Wave from a Cylindrical Mirror 



and enters a hemispherical mirror. We know that a concave mirror 

 has the power of bringing light to a focus at a point situated half-way 

 between the surface of the mirror and its center of curvature. If the 

 light comes from a very distant point, and the mirror is parabolic in 

 form, the rays are brought accurately to a focus; which means that the 

 reflected wave is a converging sphere, — a condition the opposite of that 

 in which spherical waves start in the focus of such a mirror. If, 

 however, the mirror is spherical, only a portion of the light comes to 

 a focus. On examining the pictures we see that the reflected wave 

 has a form resembling a volcanic cone with a bowl-shaped crater. 



* Cylindrical mirrors have been used instead of spherical, for obvious reasons. A sectional 

 view of the reflected wave is the same in this case as when produced by a spherical surface. 



