FRESNEL'S CONSTRUCTION OF LAMPS. IGS 



to sit below or above the horizontal line, you 

 perceive but little if any of the light, yet you 

 must bear in mind that we want the rays to go 

 in a straight line to the horizon. So that all 

 that building up of rings of glass is for the 

 purpose of producing one fine and glorious lens 

 of a large size, to send the rays all in one direc- 

 tion. Here is another apparatus used to pull 

 the rays down to a hprizontal sheet of light, so 

 that the mariner may see it as a constant and 

 uniform fixed light; the former lamp is a re- 

 volving one, and the light is seen only at certain 

 times as the lenses move round, and these are 

 the points which make them valuable in their 

 application. 



There are various orders and sizes of lights in 

 lighthouses to shine for twenty or thirty miles 

 over the sea, and to give indications according 

 to the purposes for which they are required; 

 but suppose we want more effect than is pro- 

 duced by these means, how are we to get more 

 light ? Here comes the difficulty. We cannot 

 get more lighi^ because we are limited by the 

 condition of the burner. In any of these cases^ 

 if he spreading of the ray, or divergence as it is 

 called, is not restrained, it soon fails from weak- 



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