46 Dr BREWSTER on the Constrwtion of Polyzonal Lenses 



The apparatus now described, is executed in a very admir- 

 able manner for the Northern Lighthouses ; but no excellence 

 in its execution, and no care in its application, can compensate 

 for its numerous imperfections and disadvantages, which we shall 

 now particularly explain. 



1. On the Imperfection of the Material employed. Of all re- 

 flecting substances, a silver surface, not produced by hammer- 

 ing, is the best. The effect of hammering is to give different 

 densities to different parts of the hammered surface ; and as it 

 is proved *, that part of the light reflected from metals pene- 

 trates the reflecting surface, and that surfaces polished by ham- 

 mering act upon the light in a different manner from a surface 

 not hammered, and ground and polished upon pitch, it is mani- 

 fest, that the light which enters a reflecting surface of unequal 

 density, or upon which that surface produces a physical change, 

 will not be reflected in lines determined by the form of the re- 

 flecting surface itself, but will be to a certain degree scattered 

 in all directions. This effect will be understood by examining 

 Fig. 7., where ABDC is the silver-plate highly magnified, and 

 CDFE the copper, the intersecting arches shewing the effect 

 produced by hammering. 



2. On the Imperfections of the Surface. The imperfections of 

 the external surface of the present reflectors, arises from two 

 causes : 1st, From its being a surface produced by hammering ; 

 and, 2dly, From its being covered with innumerable scratches 

 and circular lines. From the first of these causes, the surface 

 cannot possibly reflect a diverging pencil of light into a parallel 

 pencil, even if the general surface were mathematically exact. 



_ 



* See Art. OPTICS, Edinburgh Encyclopaedia, vol. xv. p. 607. ; and BIOT'S 

 Traite de Physique, torn. iv. p. 579. 



