Wigham — Lenses for Lighthouse Illumination. 531 



the full power of the lens, followed by two red flashes each of equal 

 intensity to the white flash, would he the characteristic appearance 

 of the light, and as a further distinctive mark, these flashes, both 

 red and white, might, if required, be broken up into groups. The 

 double barbed arrows in B, Plate IX., show the white beams, and 

 the single barbed arrows, the red beams. In this case the appa- 

 ratus is six-sided. 



As in former cases I tried this hexagonal light practically 

 with great care. With the consent of the Commissioners of Irish 

 Lights, I have within the last few weeks placed it at the Howth 

 Baily Experiment House, and critically watched its performance 

 from Salthill, with the result exactly as I have mentioned. 



Important as are the features of the light which I have been 

 describing, by far the most important plan which the admirable 

 idea of Sir George Stokes suggested is that of which I now come 

 to speak of. In it there is no blank face, all the light is utilised 

 and transmitted evenly to the horizon. It is shown in Plate X. 

 The lenses are placed so as to form squares, not hexagonal figures, 

 as in the previous instance. All the faces of the squares transmit 

 beams of light of equal intensity, following each other at intervals 

 determined by the rate of rotation which may be given to the 

 apparatus, say of 20, 30, 40, 50, or 60 sees., and the light of 1 

 blending with that of 3, 2 with 4, 5 with 7, and 6 with 8, pro- 

 duce a regular succession of beams with an illuminating power 

 never before attempted in lighthouse practice. 



This model will show the details of this particular light which 

 differs from the others in the grouping of its lenses and in having 

 a greater breadth and duration of beam. It must be borne in mind 

 that this light, as also the others which have been under our con- 

 sideration this evening, may be used as biform, triform, or quadri- 

 form, i. e. that 4, 6, or 8 burners may be employed conjointly to 

 pour their light through their respective lenses upon the sea, and 

 that it is only necessary to use one burner for ordinary weather, 

 the other 3, 5, or 7 being available at a moment's notice on the 

 occurrence of fog ; and although during the continuation of the fog 

 the expense is of course considerable, yet I think it is obvious that 

 during the dangers which fogs involve no reasonable expense should 

 be considered too great to protect life and property ; and as fortu- 

 nately the aggregate duration of fog during the year is but very 



