Wigham — Lenses for Lighthouse Illumination. 529 



originally devised, and explain Sir Robert Ball's remark as to group- 

 flashing : — Plate IX., elevation and plan A, shows, as I have 

 already said, that the light is emitted from the two sides, 

 and not from the two ends of the apparatus. This diagram, 

 and thtse other two, represent the ground plan of the 

 apparatus They are so made that I can revolve them exactly 

 as they v»ould revolve if in actual use. The arrows represent 

 the rays cf light forming the beam emitted from each side 

 of the apparatus. I will bring the face which is now 

 illuminating the ship marked on the black-board right round to 

 the opposite side, when the ship will receive the light from the 

 opposite fact, and the whole horizon will have been visited by the 

 light, by the complete revolution of the apparatus. In order that 

 the light fron one face should be replaced by the light from the 

 other face vithout too great delay, say within a period not 

 longer than Due minute (the longest interval of darkness now 

 allowed in lighthouses), it is necessary to rotate the apparatus 

 rapidly. Butthis rapid rotation reduces the duration of each flash; 

 that is to say the light from each face passes the mariner's eye 

 very quickly, ;ay in about seven and a-half seconds. This length 

 of flash is quie long enough to enable him to have a good look at 

 the light anc take his bearings from it ; but it is hardly long 

 enough to pemit of the beam being broken up into the minor 

 beams or gro.ps of flashes which I have described — such as are 

 used in the g-eat lighthouses of Galley Head and Mew Island. 

 In other word, this two-sided apparatus, while it forms the most 

 powerful reveving light ever seen, is not suitable for cases where 

 such group-flshes are required. 



And now . come to the last and most important development 

 of the systen of lighting which I have the honour of introducing 

 to your notie. Last autumn I had a visit from Professor Sir 

 George Gabiel Stokes, Bart., M.P., President of the Eoyal 

 Society. He;aid he had been told by my brother what the double 

 quadriform l^ht meant, and asked to have it fully explained to 

 him. This vas done by means of a model which happened to be 

 at hand. Aier some consideration Sir George said that his con- 

 ception of he double quadriform light had been somewhat 

 different ; an. he sketched out with a pencil what had been bis 

 idea. On th occasion of a subsequent visit I was able to show him 



