56 Dr BREWSTER on the Construction of Polyzonal Lenses 



in Fig. 10., in strengthening and widening the main beam AH, 

 BR. But, though the reflector throws much light into the 

 beam, it reflects it in a very imperfect manner, from the causes 

 which we have already explained. In the lens, on the contrary, 

 the light is refracted into the beam by a highly polished and 

 regular surface ; and when we consider, that, in a lens, three feet 

 in diameter, the distance LC is three feet, while in the reflector, 

 the distance LC is little more than three inches, we must see at 

 once how peculiarly the lens is adapted to collect the cone of 

 rays LAB into a dense and regular beam, capable of penetrat- 

 ing space, and forcing its way through the fogs and mists of the 

 ocean. 



From the nature of a parabolic reflector, we are prevented 

 from using a very powerful lamp, and hence a common argand 

 burner is the only light which has been hitherto used in Great 

 Britain. The proximity of the focus to the back part of the 

 mirror, renders it impracticable to increase the flame, without 

 at the same time diminishing the parallelism and density of the 

 reflected column. In the case of the lens, however, we may 

 use the powerful lamp recommended by Count RUMFORD with 

 2, 3, 4, or even 5 and 6 concentric wicks ; and we can thus 

 throw a much greater quantity of light into the refracted beam, 

 than we can possibly throw into the beam formed by reflection. 

 In the present system of illumination, it is out of our power to 

 increase the light in cases of emergency, when the lighthouse 

 ceases to be visible at short distances ; but, in the system of il- 

 lumination by lenses, we may increase the light tenfold of what 

 is necessary in a favourable state of the atmosphere. 



In this comparison, we have supposed, that all the rays which 

 flow from L, Fig. 10. are lost, excepting those between LA and 

 LB ; but while we retain the lens AB, we can enlarge the cone 

 of rays LAB, by placing a small lens between L and C, and 

 we can increase its intensity, either by throwing back through L 



