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LII. 



AN IMPROVED METHOD OF USING ANNULAR LENSES FOR 

 LIGHTHOUSE ILLUMINATION. By JOHN R. WIGHAM. 

 (Plates IX. and X.) 



[Eead April 16, 1890.] 



On a previous occasion I brought under the notice of this Society 

 an arrangement which I had just devised for superposing the 

 lenticular apparatus used in lighthouses, so as to obtain a greater 

 light than could be derived from the apparatus in its original 

 form. Since then, that arrangement has been adopted both in 

 England and Ireland in several important lighthouses, and has 

 given great satisfaction to shipowners and seamen. 



I have now to describe a still further improvement in light- 

 house illumination, involving a still greater departure from the 

 old system. That this description may be intelligible, I must 

 briefly recapitulate and explain the steps which have led up to this 

 improvement. I have here a complete segment of the lenticular 

 apparatus used in lighthouses of the first order, such as Howth 

 Baily. It will be seen, when I light one of my lighthouse gas 

 burners in its focus, that the light proceeds from the whole of its 

 surface. The apparatus is divided into three parts, the great central 

 belt from which 75 per cent, of the light is transmitted, and the 

 top prisms and bottom prisms which transmit the remaining 25 

 per cent. 



It was because this central portion of the apparatus is so much 

 more powerful in the transmission of light than the top and bottom 

 prisms combined, that I conceived the idea of discarding these 

 prisms altogether, usiug instead the central parts only, superposed ; 

 say two, three, or four of them, giving respectively 50 per cent., 125 

 per cent., and 200 per cent, more light than in the old system. 

 This increased light is of course only obtained when the two, 

 three, or four burners are lighted. Contrary to what might be 

 supposed, there is no great increase of annual cost by the use of 



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