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LECTURES TO SCIENCE TEACHERS. 



to give these lenses a sufficient diameter, he further con- 

 structed them in the form known as that of echelonned 

 lenses, invented first it is said by Buffon, and after- 

 wards independently by Fresnel himself. We have here 

 the original lenses, constructed by Soleil, the great optician 

 of his day in Paris, under Fresnel' s orders. You will per- 

 ceive that the larger lenses are square, the corners being 

 made up in a similar manner by portions of an echelonned 

 lens. 



Fig. 1 represents one of these lenses in horizontal 



Fin. 1. 



section. You will notice that the form of the lens is pro- 

 duced by revolving a section round a horizontal axis. It 

 is the form adopted for revolving lights, when the lantern 

 is of polygonal form. As the lantern revolves each time 

 that a face passes any particular direction in azimuth, 

 a flash is emitted through the lens, followed by almost 

 total darkness when one of the angles comes round into t he 

 same direction. These lenses are set in frames slightly 

 oblique to the frame of the lantern, so that in the case of 

 a complete beehive, such as this, the flash is prolonged, t he 



