60 Dr BREWSTER on the Construction of Polyzonal Lenses, 



shewing double and single stationary lights, exhibited from se- 

 parate lighthouse towers. This description of lighthouse is suf- 

 ficiently characteristic : it is, however, not only expensive, but, 

 from the frequent repetition, such lights have at length become 

 so general, as to be no longer a distinguishing guide to the ma- 

 riner. The next idea which suggested itself, was the revolving 

 light, exhibiting the alternate effect of light and darkness, by 

 the periodical revolution of a, frame or chandelier with reflectors, 

 kept in motion by machinery. The revolving light has also 

 been constructed as single and double ; and even treble revolv- 

 ing lights, as at the Casket Rocks, in the British Channel. 

 But this mode, from the increasing number of lighthouses, it 

 has also been found necessary to vary ; and revolving lights are 

 now distinguished from each other by shades of glass stained of 

 a red colour, which are interposed between the eye of the spec- 

 tator and the reflector. Upon the first suggestion of this plan, 

 it was expected that a great range of colours might be made use 

 of; but after many trials with glasses coloured red, green and 

 blue, and also by means of coloured fluids, introduced between 

 plates of white glass, it has been found that red shades only were 

 calculated to answer the purpose effectually, of distinguishing 

 and characterising sea lights *. To complete the lighting of the 

 coasts of Great Britain and Ireland, however, many lighthouses 

 must still be erected ; and the distinguishing of the new light- 

 houses from those already in use, becomes an object of the first 

 consideration with persons engaged in these useful and import- 

 ant works." 



* In his Account of the Bell Rock Lighthouse, p. 322., Mr STEVENSON adds, 

 " that, after the most full and satisfactory trials, the red colour was found to be the 

 only one applicable to this purpose. In tolerably clear weather, the light of one re- 

 flector tinged red was easily distinguishable, at the distance of eight or nine miles ; 

 while the other colours rendered the light opaque, being hardly distinguishable to 

 the naked eye at more than two or three miles. 11 



