for the purposes of Illumination in Lighthouses. 53 



removed from them a great portion of the solid glass, and that, 

 as the surfaces of the glass which is left, are parallel to the sur- 

 faces of the glass which is removed, the rays of light will suffer 

 nearly the same refractions in the one lens as in the other. Let 

 AC B bm A, Plate IV. Fig. 8., for example, be the section of a 

 large solid lens, from which the great mass of glass efg acb hi 

 k C e has been removed, the polyzonal lens which is left, will re- 

 fract light nearly in the same manner as the solid lens, in con- 

 sequence of the surfaces fg and a c b being parallel to e C k. A 

 ray of light FC falling on the solid lens at C will be refracted into 

 the line C n, and will emerge in the direction n R. In the poly- 

 zonal lens, the ray F c will be refracted at c into a line c m, nearly 

 parallel to C , and will consequently emerge at z, in a direction 

 R z, nearly parallel to n R. I have said nearly, because there 

 is a slight difference between the refraction in the two cases, but 

 this difference, as will afterwards be seen, is in favour of the po- 

 lyzonal lens, which is actually a more perfect lens than the so- 

 lid one. The following are the advantages of the new lenses, 

 compared with those of the common form. 



1. The polyzonal lenses are much more transparent than 

 common ones made of the same glass. As the finest glass 

 has a decided colour above certain thicknesses, and as the tran- 

 sparency of different masses is inversely proportional to their re- 

 spective thicknesses, the polyzonal lenses must, from their very 

 nature, have a superior transparency to common ones made of 

 the same glass. 



2. As it has been hitherto found impracticable to cast large 

 lenses free of veins, flaws and impurities, which scatter and ob- 

 struct the refracted light, the formation of them, in separate 

 zones and pieces, enables us not only to construct them of pure 

 and homogeneous glass, but to make them of a size which has 

 been hitherto deemed impracticable. When it is impossible to 

 obtain 300 Ib. of good homogeneous glass for a solid lens, it may 



