364 POPULAR SCIENCE MONTHLY. 



on the film, was accordingly adopted. Three complete sets of drawings, 

 to the number of about one hundred each, were prepared for three sepa- 

 rate cases of reflection; — viz.: the entrance of a plane wave into a 

 hemispherical mirror, the passage of a spherical wave out from the focus 

 of a hemispherical mirror, and the multiple reflection of a spherical 

 wave inside of a complete spherical mirror. Special methods were 

 devised for simplifying the constructions, and much less labor was 

 required in the preparation of the diagrams than one would suppose. 

 The results fully justified the labor, the evolutions of the waves being 

 shown in a most striking manner. These films I exhibited before the 

 Eoyal Society in February last, and a more complete description of the 

 manner of preparing them may be found in the Proceedings of the 

 Society. 



A portion of one of these series is reproduced, about one in four or 

 five of the separate diagrams being given. The series runs from left 

 to right in horizontal rows. When projected on the screen, the spheri- 

 cal wave is seen gradually to expand from the focus point, like a swell- 

 ing soap bubble; it strikes the surface, and the bowl-shaped echo bounces 

 off and follows the unrenected portion across the field; these two por- 

 tions are then reflected in turn, and the curiously looped wave flies back 

 and forth across the mirror, changing continuously all the time, and 

 becoming more complicated at each reflection. These diagrams should 

 be compared with the photographs shown in the fourth series. 



One must not suppose that these beautiful forms exist only in 

 the laboratory. Every time we speak, spherical waves bounce off the 

 floor, ceiling and walls of the room, while in any ordinary bowl or basin 

 the curious crater-shaped echoes are formed. Glance once more at the 

 wave surfaces produced within a hollow sphere, and try to imagine the 

 complexity of the aerial vibrations caused by a fly buzzing around in an 

 empty water-caraffe! The photographs enable us to realize what is 

 going on around us all the time — this our perceptions are fortunately 

 too dull to perceive. Life would be a nightmare if we were obliged to 

 see the myriads of flying sound waves bounding and rebounding about 

 us in every direction, and combining into grotesque and ever-changing 

 forms. It is just as well, on the whole, that the light of the electric 

 spark and the delicate optical device of Toepler are necessary to bring 

 them into view. 



