66 Dr BREWSTER on the Construction of Polyzonal Lenses, 



the cone of rays near the lamp, and colour the whole of the illu- 

 minating column. 



3. The. two methods of forming distinguishing lights, which 

 have now been described, might in some cases be advantageous- 

 ly combined, so that in places where lighthouses are numerous, 

 we may, at little additional expence, produce many well-marked 

 variations in revolving lights. 



In particular cases, where the lighthouses are exposed only 

 on one side to the ocean, a motion of the apparatus through the 

 arch of a circle is all that is necessary, and there are situations 

 where a slight angular motion of the illuminating column in a 

 vertical plane might be desirable. 



V. On the occasional exhibition of powerful Lights in Light- 

 houses. 



In the present system of illumination, no provision whatever 

 has been made for the occasional exhibition of intense lights, 

 when the atmosphere is so hazy and foggy as to absorb entire- 

 ly, at moderate distances, all the rays which proceed from 

 the reflectors. At the Bell-Rock Light-house, two large bells, 

 each weighing twelve hundred weight, are tolled night and day 

 during foggy weather, so as to warn the mariner of his approach 

 to the rock. This contrivance is certainly better than none, 

 though there are cases in which it may mislead the mariner to 

 his ruin. 



No fact in physics is better established, than the inability of 

 the ear to judge of the direction of sound ; and, indeed, the 

 whole deception of the ventriloquist is founded upon this fact. 

 In some conditions of the atmosphere, the sailor may err in his 

 judgment of the direction of the sound several points of the com- 

 pass, and he may thus be cast on the very rock which, under 

 the guidance of other data, he might have avoided. 



