ON LIGHTHOUSE APPARATUS. 



317 



Fresnel further improved the lens by grinding the rings to different 

 centres, and so practically got rid of spherical aberration altogether. 

 Then, Fresnel, instead of using a number of lenses, and with separate 

 flames, used one central flame and surrounded it by a number of those 

 thin annular lenses, so that whenever the axis of one of those large 

 lenses is pointed to a distant observer he gets a flash of light, and 

 when it passes away he loses that light. But in order to save the light 

 which passes over the top of this apparatus, Fresnel had recourse to 

 two agents, namely, smaller lenses inclined to the horizon, and plane 

 silvered reflectors also inclined. By this compound arrangement the 

 light was thrown out above the main lenses in horizontal beams paral- 

 lel to those coming from the large lenses below. Such was Fresnel's 

 revolving light, figs, i & 2, but he did not stop there. He introduced 



< . 



Figure I and 2. 



L principal lenses, / inclined lenses, M plane mirrors. 



