The light of Skerryvore was exhibited to the mariner on the night of the 1st February 1844, in terms of the Statutory Notice, which will be found in the Appendix. The light is revolving, appearing in its brightest state once in every minute of time. It is elevated 150 feet above the sea, and is well seen as far as the curvature of the earth permits; it is also frequently seen as a brilliant light from the high land of Barra, a distance of 38 miles. The apparatus consists of eight annular lenses (of the first order, in the system of AUGUSTIN FRESNEL), of 36·22 inches focal distance, revolving round a lamp with four concentric wicks, and producing a bright blaze when each lens passes between the lamp and the eye of a distant observer. Above those lenses are placed eight pyramidal lenses of 19·68 inches focal distance, inclined at 50° with the horizon and combined with eight plane mirrors, inclined in the opposite direction at 50° with the horizon. By this arrangement, that part of the light from the lamp which would otherwise escape uselessly beyond the great lenses, upwards into the sky, being parallelized in its passage through the smaller lenses and falling on the mirrors, is finally projected forwards in horizontal beams, so as to aid the effect of the light. Those lenses and mirrors, however, instead of having their axes in the same vertical plane with the axes of the principal lenses, are inclined about 7° horizontally to the right hand, and by that deviation produce small premonitory blazes, which, blending with the beams of the larger lenses, tend in some measure to lengthen the duration of the impression on the eye. So far the apparatus of the Skerryvore Lighthouse is identical in its general arrangements with that of the Tour de Corduan, and differs only in the superior workmanship of the lenses and the machinery, which the experience of more than twenty years has brought about, since FRESNEL designed that light in 1822. Instead, however, of employing curved mirrors, as has been done at Corduan, to collect the light which would otherwise escape below the lenses and, at the same time, to send it to the horizon, I determined to put in practice a plan which I had long contemplated, of placing totally reflecting zones below the lenses, similar in construction to the zones of the small Harbour Light Apparatus of the fourth order, which was also invented by FRESNEL. This was finally carried into effect, agreeably to the design of M. LEONOR FRESNEL, his brother, with whom I had corresponded on the subject. In the subsequent pages of this volume, I intend to make some observations explanatory of the principles and arrangement of the various optical instruments employed in Lighthouses; and as that will afford me a more convenient opportunity of describing the nature and properties of the _totally reflecting zones_, I shall forbear in this place to enter into further details as to the construction or action of the apparatus at the Skerryvore.