DIFFERENT FORMS. 189 



The rays of light from the charcoal points to the left are 

 brought into a parallel beam as they leave the lens on the 

 right. 



In this case, the rays, diverging first from a point, become 

 parallel, which is the reverse of the case of the sun-glass, 

 in which parallel rays that is, rays sensibly parallel on ac- 

 count of the sun's great distance are made to converge to 

 a point. All refraction is in this way reciprocal. Rays 

 subjected to refraction will always follow the same track 

 in this sense, namely, that if they enter the glass on the 

 right side, for example coming in a certain direction, and 

 go out on the other side in a certain different direction, 

 then, if the motion is reversed, and the rays of another 

 beam come in on the left as the former went out, they will, 

 on refraction, in the same lens, go out on the right precise- 

 ly as the former came in. 



It is plain that a lens modified on Fresnel's system, as 

 above described, might be made of a circular form, as 

 usual, in which case several sets of them would be required, 

 forming diiferent faces, to be presented toward different 

 quarters of the horizon. Or the lens might be made annu- 

 lar, with a broad convex surface in the centre, and narrow 

 ones in rings above and below. This last arrangement is 

 shown in a simple form in the engraving on the following 

 page, which represents a signal, lantern such as is used on 

 board ships. 



These engravings are copies of those which Lawrence 

 gave to John to put into his note-book, as illustrating in a 

 simple form the fundamental principle of Fresnel's idea. 

 John afterward found, when he came to visit light-houses 

 on the coasts of France and England, that, in carrying the 

 idea into practical effect, a great number and variety of 

 most elaborate and complicated arrangements were made. 

 When he Avent inside of some of the large lanterns and 



