The distinctions are, in fact, only _four_ in number, viz.: Fixed; Fixed, varied by flashes;[74] Revolving, with flashes once a minute; and Revolving, with flashes every half minute. To those might be added, Revolving, with bright periods once in two minutes, and perhaps _Flashing_ once in _five seconds_ (as introduced by me at the Little Ross, but I cannot say with such complete success as would induce me to recommend its general adoption). My own experience would also lead me to reject the distinction called “Fixed, varied by flashes,” which I do not consider as possessing a marked or efficient character. [74] The “Feu fixe, varié par des éclats,” or “Feu fixe, à courtes éclipses,” of Fresnel. ~Comparison of Dioptric and Catoptric Apparatus for Revolving Lights.~ Having thus fully described the nature of the catoptric and dioptric modes of illuminating lighthouses, I shall next compare the merits of both systems, with a view to determine their eligibility in revolving or in fixed lights. Repeated experiments were made at Gullan-hill, which is distant from Edinburgh about fifteen miles, during the winters of 1832 and 1833, under the inspection of the Commissioners of Northern Lights, the result of which was, that the light of one of the great annular lenses used in the revolving lights of the first order, was equal to the united effect of about eight of the large reflectors employed in the revolving lights on the Scotch coast. It may be said, however, that the dia-catoptric[75] combination of pyramidal lenses and plane mirrors of Corduan, adds the power of more than two reflectors to the effect of the great lens; but it ought to be remembered that in the French lights, this additional power is used only to compensate for one of the defects of the system by lengthening the duration of the flash, and therefore contributes, if at all, only in a very indirect manner, to render the light visible to the mariner at a greater distance. M. FRESNEL found, from the smaller divergence of the lens, that the eclipses were too long and the bright periods of the revolution too short; and he therefore determined to adopt the horizontal deviation of 7° for the upper lenses, with a view to remedy this defect. Assuming, therefore, that it were required to increase the number of reflectors in a revolving light of three sides, so as to render it equal in power to a dioptric revolving light of the first order, it would be necessary to place eight reflectors on each face, so that the greatest number of reflectors required for this purpose may be taken at _twenty-four_. M. FRESNEL has stated the expenditure of oil in the lamp of four concentric wicks at 750 grammes of colza oil per hour; and it is found by experience at the Isle of May and Inchkeith, that the quantity of spermaceti oil consumed by the great lamp, is equal to that burned by from fourteen to sixteen of the Argand lamps used in the Scotch lights. It therefore follows that, by