1SS2.] "^'^^ [EcUly. 



eel. Let the projectiles from 5 have the common velocity «'. Such of these 

 projectiles as succeed in passing through the apertures of b will impinge on 

 c at points between its apertures, in case c be placed at a proper distance 

 from h. Let the surface of o which faces b be perfectly reflecting, and let 

 the parts between its apertures be either concave or a series of inclined 



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planes so directed that each of the projectiles on rebounding will pass back 

 through one of the apertures in b. When the velocity «' of the projectiles 

 is large compared with that of the screens u, the projectiles can be made 

 to return through b very nearly perijeudicularlj'', either by returning each 

 projectile through that aperture from which it started or through some 

 following one. 



