Wigham — Lenses for Lighthouse Illumination. 527 



had been obtained by placing lenses vertically over each other 

 might also be gained by placing them side by side. This led to 

 an entirely novel arrangement of the lenses : not only their being 

 placed side by side, but formed into a figure having only two 

 great illuminating faces and two dark ends. In another respect 

 also it was novel. Under the old plan the lenses are caused to 

 revolve round a central burner : but in this case it was necessary 

 for the burners as well as the lenses to revolve, so that the precise 

 focal relation between them might be constantly maintained. The 

 use of a lute of water or mercury enabled me to do this, and at 

 the same time to provide a constant flow of gas or oil to the 

 burners or lamps. One of the models on the table shows these 

 arrangements. It also shows that in this new plan there is 

 sufficient space to enable ventilating tubes, gas-pipes, &c, to be 

 so placed as not to obstruct any portion of the light, whereas in 

 the old form of apparatus these tubes present some amount of 

 obstruction to the light in certain bearings. 



I tested this form of light practically, first in the workshops of 

 the Company with which I am connected, and subsequently (with 

 the consent of the Commissioners of Irish Lights) in the Experi- 

 ment House at Howth Baily. The result was precisely what I had 

 anticipated, namely, the illuminating power of the beam was 

 exactly double that of the quadriform. I asked Sir Howard Grubb, 

 whose practical acquaintance with optical science confers such great 

 value upon his opinion, to examine it critically. He kindly 

 acceded to my request ; and the sum total of his report was a 

 distinct corroboration of the opinion that it possesses double the 

 power of an ordinary quadriform light. Professor Barrett also 

 favoured me by making repeated observations respecting this 

 double quadriform light. The weather was particularly favourable 

 for his observations, the result of which he conveyed to this Society 

 in the form of an interesting Paper read at one of its scientific meet- 

 ings, and since printed {Proceedings, Yol. v., p. 74). He mentioned 

 specially a most important fact, viz. that while he was looking at 

 the light a fog came on, and yet he was able to see it though six 

 miles distant, while at the same time the full fog power of the 

 Baily lighthouse was invisible. Sir Eobert Ball, the Astronomer 

 Eoyal, also gave me his testimony to the beauty and the splendour 

 of the light when he accompanied the late Earl of Meath and 



