Care must be taken, in preparing the leathern valves of the pump-box or chamber, shewn in [Plate XXII.], that they be neither too flaccid from largeness nor too tense from smallness; and also that, after being fitted, they draw no air. To remove the old valves and replace them by fresh ones, is a very simple process, more especially when a proper die or mould is used, which at once cuts the kid-leather, of which the valves are formed, to the required size and squeezes them into the proper shape. In [Plates XX.], [XXI.], [XXII.], [XXIII.], [XXIV.], and [XXV.], the most minute details are given as to the clockwork, pumps, burners, and flame of the great lamp.[71]
[71] See also M. Leonor Fresnel’s Instructions sur l’organisation et la surveillance du service des Phares et Fanaux de France. Paris, 1842, pp. 12, 13, 14, and 15.
The focal point for the lenses and refractors is in the centre of the flame and on the level of its brightest film, as shewn in [Plate XXV.] Choice of Focal Point for various parts of the apparatus. The choice of a focus for the zones naturally formed a most important practical consideration in their arrangement; and the judicious remarks of M. Leonor Fresnel on that subject, already noticed, would alone have induced me to discard my former calculations in favour of his. For the upper zones, M. Fresnel had adopted a point in the centre of the flame 10 millimètres above the focus of the lenses, so that all the light below that point necessarily falls between the horizon and the Lighthouse; but for the lower zones, it was necessary, owing to their arrangement for convenience in a cylindric form, to adopt a separate focus for each zone in the direction of the centre of gravity of that part of the flame which would light each zone. In this manner ([fig. 86]) the foci of the zones recede upwards from a to f in proportion to the depression of the zones a, b, c, d, e, f, so that the line joining each zone and its focus, must revolve as a radius vector round some point O between them. The details of this arrangement are shewn in [Plate XVIII.]; and are also given in the [Table] of the Catadioptric Zones in the Appendix.
Fig. 86.
Fig. 87.
In the arc next the land, in fixed lights, a great loss of light ensues from the escape of the rays uselessly in that direction. So far back as 1834, I suggested the Application of Spherical Mirrors to fixed Dioptric Lights. placing a segment of a spherical mirror, with its centre of curvature coincident with F the focus of the system, so that the luminous pyramid MFM, of which the mirror MM forms the base, might be thrown back through the focal point and finally refracted into such a direction as to contribute to the effect of the lens QAq in seaward and opposite arc. In the diagram ([fig. 87]), r r indicate rays proceeding directly from F; r′ r′ rays reflected from MM through F, and finally refracted at QA q; and r″ r″ is the beam compounded of both. In the best glass-silvered mirrors, this accession of light would amount to nearly half of the light incident on them. In such an arrangement, a considerable radius is desirable to decrease the amount of aberration produced by a large flame. In the case of revolving lights of the first order, the radius would, of course, be limited to somewhat less than three feet, which is the focal distance of the lenses, between which and the focus, the reflecting segment must be placed; but in fixed lights, the lantern is the limit of radius, so that a focal length of five feet ten inches may be obtained. M. François ground some beautiful mirrors of three feet radius, which were afterwards tinned by his successor, M. Letourneau, by a new process discovered by himself;[72] and that gentleman is at present engaged in the construction of reflecting spherical segments 1200mm. square (about 16 superficial feet), to a radius of 1770mm. (5 feet 10 inches), which subtend a vertical arc of about 40°.
[72] See notice of a similar process practised about the year 1750 by Mr Rogers of London, ante, [p. 240].
Arrangement of Dioptric Apparatus. The arrangements of the dioptric apparatus in the lightroom will be more fully understood by referring to the Plates.