foi~ the purposes of Illumination in Lighthouses. 55 



hand side of the lenses, the aberration will be greatly less than 

 it is in the solid lenses, Fig. 1 . and Fig. 4. 3. When the zones 

 are placed, as in Fig. 1 . and Fig. 4., the aberration may be cor- 

 rected by diminishing the curvature of the zones, as they recede 

 from the central lens, or by varying the inclination of their sur- 

 faces to the axis of the lens, till the middle line of each zone is 

 nearly in the surface of a hyperboloid. By any of these ar- 

 rangements, it is easy to construct the lens, so that parallel rays 

 shall be collected within a space not exceeding the magnitude 

 of the flame from which the parallel beam of light is to be ob- 

 tained, which is all that is required for the purposes of light- 

 houses. But, when the lens is to be used as a burning instru- 

 ment, the accurate correction of the spherical aberration is, as 

 Mr HERSCHEL has found, a matter of the first importance. 



Having thus described a method of constructing lenses su- 

 perior in transparency, in homogeneity of substance, in size, and 

 in their action upon light to solid lenses, we shall now point out 

 their superiority to hammered parabolic reflectors for the pur- 

 poses of lighthouses. 



Let AB, Plate III. Fig. 10., be a lens which forms a parallel 

 beam of light AR, BR, by means of a lamp at L placed in its 

 focus. By comparing Fig. 6. with Fig. 10., it will be seen that 

 the reflector ACB, Fig. 6., throws into the parallel beam A m 

 B n, all the light which radiates from L, excepting what is con- 

 tained between LA and LB ; whereas the lens AB, Fig. 10., 

 throws into its parallel beam only what is contained between 

 LA and LB. The loss of light, however, in the reflector is 

 more than one-half of what falls upon it, while in the lens it is 

 only about one-tenth. This circumstance alone compensates, to 

 a certain extent, for the smaller portion of the sphere of rays 

 which falls upon the lens ; and it will be afterwards seen that we 

 can actually avail ourselves of the rest of the sphere of light 



