64 Dr BREWSTER on the Construction of Polyzonal Lenses, 



light, and ten eclipses, during each revolution of the appara- 

 tus *. 



2. In order to produce distinguishing lights, by altering the 

 colour of the rays, it is necessary, under the present system, to 

 cover the whole mouth of the reflector with a plate of coloured 

 glass mn, Plate III. Fig. 6. two feet in diameter ; and in the 

 passage already quoted, we are informed by Mr STEVENSON, that 

 no other colour but red has been found to answer. This colour, 

 however, is the worst that can be employed, as it is the very co- 

 lour which white light assumes in passing through a dry hazy 

 atmosphere, or through a long tract of even clear air. Hence 

 occasions will often occur, when such a colour will cease to be 

 a distinctive mark of any individual lighthouse. 



When it is admitted that red shades only have been found to 

 answer the purpose of characterising sea-lights, it is a virtual 

 admission of the total incompetency of the present system of il- 

 lumination, for nothing can be more certain, than that other co- 

 lours may be introduced as characteristic of sea-lights, provided 

 the intensity of the illuminating columns is sufficiently strong 

 to allow of that additional loss by absorption, which takes place 

 in passing through various coloured media. 



* In order to render available the reflectors of the old system, the following com- 

 binations may be adopted with advantage in many cases. 



As the back part of the reflector is almost useless, an aperture two or three inches 

 in diameter may be cut away at D, Fig. 5., so as to give free passage to the cone of 

 rays FAB, which, falling upon the lens AB, will be reflected into a parallel beam 

 ARBR. 



Two reflectors CDEE', C'lVEE' may be coupled together, as in Fig. 6., so that 

 the lamp F may be exactly in the focus of each, and in this manner we shall have two 

 beams of light in place of one. 



Or we may give additional power to the reflector, as in Fig. 7. by using another 

 lamp F, and surrounding the reflector with the external zones of a lens AB, in whose 

 focus the lamp F is placed. The column of light CDRR, thrown out by the reflector, 

 will be widened on all sides by a hollow cylinder of light, whose section is ACRR, 

 DBRR. 



