638 SIR DAVID BREWSTER ON A DOUBLE HOLOPHOTE APPARATUS. 



each of which is surrounded by its feeble cylindrical ones. In this construction 

 we might dispense with the use of large built-up lenses, and employ small lenses 

 of flint-glass, which can now be manufactured at a moderate expense, and of 

 great excellence. 



The suggestion to introduce the Drummond or the electric and magnesian 

 lights as permanent lights in our lighthouses is not likely to be adopted. Oil and 

 gas light, concentrated optically into a powerful beam, has a sufficiently pene- 

 trating power in ordinary states of the weather, and more brilliant lights are re- 

 quired only in fogs and states of the weather when feebler lights cannot be seen. 



In my paper on the Illumination of Lighthouses, in our Transactions for 1827, 

 I have shown in Plate III. Fig. 1, two methods of occasionally introducing the 

 electric or other light into the central beam of a holophote, without interfering 

 with its ordinary action* If we consider the front holophote in fig. 1 to be the 

 ordinary dioptric light, the second holophote, with the electric or other light at F, 

 might be used in place of the simpler and less effective one above referred to. 



* The method of doing this by lenses, whose conjugate focus is F', Fig. 1, or by an ellipsoidal 

 mirror, one of whose foci is F, when the spherical mirror (MN, Plate III. Fig. 1, above referred to) 

 is removed, is distinctly shown ; but the note describing it was accidentally omitted by the printer. 



