50 Dr BREWSTER on the Construction of Polyzonal Lenses 



tain KATER has shewn, that the intensity of a pencil of light, af- 

 ter its rays have crossed one another in a focus, is reduced 

 nearly one-half* ; and though the cause of this is not fully as- 

 certained, yet it is obvious, that a beam of light, composed of 

 rays imperfectly reflected, crossing one another in every part of 

 its section, must, from this cause, undergo a great diminution of 

 intensity. 



In addition to the disadvantages now explained, we may 

 mention two others, which merit particular notice. 



1. The Parabolic Reflectors do not admit of any augmentation of 

 the light in cases of emergency. In dark and hazy weather, when 

 the mariner requires to be warned of his danger by the ringing 

 of bells, it would be most desirable to double, or even quadruple, 

 the intensity of the light. One reflector, however, cannot, in 

 such cases, be made to augment the effect of another, and the 

 introduction of a larger burner, in place of producing an increase 

 of light, would actually occasion a diminution of it f. It will 

 be seen, however, in the sequel of this paper, that the polyzonal 

 lenses possess this advantage in a peculiar manner. 



2. The Parabolic Reflectors are peculiarly unfit for the pro- 

 duction of distinguishing lights. In order to form a distinguish- 

 ing light, by difference of colour, it is necessary to interpose a 

 plate of red glass, two feet in diameter, in front of the reflector. 

 This method is not only an expensive one, but it is very limited 

 in its resources. In the case of a lens, a piece of glass a few 

 inches square is sufficient, and from this cause we can avail our- 

 selves of various coloured media, which could not be used in the 

 present system. 



* See EDINBURGH ENCYCLOPEDIA, Art. OPTICS, vol. xv. p. 67. 

 f- A burner with two concentric wicks should be immediately introduced into 

 the lamps now in use. 



