530 Scientific Proceedings, Royal Dub/in Society. 



an arrangement which I had worked out from his idea : Plate IX., 

 elevation and plan B, explains it. It will be seen that this form of 

 double-revolving light is perfectly applicable to group-flashing 

 lights ; for, by the arrangement which Sir George suggested, the 

 apparatus, instead of being two-sided, is made four-sided ; and thus 

 its speed of rotation is reduced by one-half, and the duration 

 of its flash increased in the same proportion. Instead of the 

 lenses being placed side by side in the same plsne as in 

 A, Plate IX., they are placed in the position shown in B, 

 Plate IX. This model will further assist in explaining this 

 form of light. By the diagram shown, the light procoding from 

 lens I. unites with that of lens III., that from II. with that 

 from IV., that from V. with that from VII., and that from VI. 

 with that from VIII., making, as I have just said, t four-sided 

 optical apparatus. This apparatus will not only coivey to the 

 mariner a light of double the power possessed by any other light- 

 house light, but linger in his eye for a period sufficidtly long to 

 admit of its being broken up into minor flashes if requied. Besides 

 this, we have in the arrangement a light having a disinctive char- 

 acter of its own ; for as the apparatus rotates, it wi] transmit to 

 the observer two brilliant flashes, succeeding each othir at intervals 

 of say half a minute, followed by a period of darknes of say one 

 minute, to be again followed by two brilliant flashes ]alf a minute 

 apart, as before, and so on continuously. It will be prceived that 

 in this arrangement four lenses out of every tier fre dispensed 

 with ; therefore the first cost of the apparatus is but snail in com- 

 parison with the enormous amount of light given to ue sailor, and 

 it is this suppression of lenses that causes the characteistic appear- 

 ance of the light, namely, two flashes followed by agiven period 

 of darkness. This peculiar arrangement lends itself m6t admirably 

 to the cases in which it might be thought desirable D use colour 

 as the distinguishing lighthouse characteristic. Uider the old 

 system, when red glass is employed, it is necessary i> reduce the 

 power of the annular lens through which the white l^ht is shown 

 to one-half its normal power in order to equalise the i>d and white 

 beam, so that they may always be seen of equal intensty, and thus 

 the actual power of the lighthouse is reduced by one-blf . Under 

 this new system there need be no curtailment of the jnnular lens, 

 but its full power would be utilised, and a brilliant "Aite flash of 



