150 Scientific Proceedings, Royal Dublin Society. 



gives effect to the whole of the light of the burners of large dia- 

 meter which are used under the gas system ; hence the illuminating 

 power of the lighthouse is enormously increased. The same applies 

 to the trilateral form. 



So long ago as 1872 I advocated in this room the great advan- 

 tages which large lenses would give if used with the large gas 

 burner which I had then introduced, and subsequently, in papers 

 read before the British Association and elsewhere, I continued my 

 contention, but it was not till 1885, when the eminent Engineers 

 to the Board of Northern Lighthouses, Messrs. Stevenson, repre- 

 sented the matter to the authorities at South Foreland that an 

 experiment was permitted to be made there with a large lens of 

 long focal range. So successful did it prove that similar large 

 lenses have since been placed at several important lighthouses, 

 notably at Tory Island. 



Last year I read a paper before this Society, explaining a 

 system of double lenses which, by the use of two burners, would 

 give precisely double the illuminating power of Tory Island ; 

 mentioning the refusal of the Board of Trade even to give it a 

 trial. Since that time I have been in communication with the 

 eminent optical engineers, Messrs. Barbier & Co., of Paris, and 

 while the advantages of the double system are undoubted, yet it 

 was evident that if lenses of sufficient size could be made to give 

 a flash equal in power to the compound flash of the double system 

 much economy would result. At my request, therefore, Messrs. 

 Barbier designed lenses for the quadrilateral and trilateral systems 

 to which I have above referred. Each of the lenses of the quad- 

 rilateral has an axial intensity of about 800,000 candles, or more 

 than twice that of the great lenses which I erected in triform for 

 the Irish Lights Commissioners at Tory Island. A still more 

 powerful light may be obtained from the trilateral apparatus, each 

 lens of which has an axial intensity of about 1,000,000 candles. 



On a future occasion I hope to have the pleasure of explaining 

 the details of these systems of lenticular arrangement, and will only 

 now repeat that the illuminating effect of the quadrilateral is about 

 2| times that of Tory Island, and of the trilateral more than 3 

 times, an increase of power which may be of incalculable benefit 

 to navigation. 



I will ask you also to remember that if with lenses of this 



