624 LIGHTHOUSE Condens- Condensing Octant. The central fixed apparatus bb (fig s. 50, 51) ing with spherical mirror dd throws its rays directly over the angle of octant. 45 pgp, while the supplemental rays fall upon the straight condens- FIG. 47. Side Elevation. ing prisms p, each of which spreads the incident rays parallel to the corresponding rays in the central angle pgp. In this way the whole of the front hemisphere of rays is parallelized in the vertical vertically upwards, when, falling on the expanding prisms gfe, it also is finally spread over the required arc. Thus the whole light is condensed and distri buted with strict equality over the 45 by means of six different optical agents. Two apparatus of this kind form the leading lights to the River Tay. Condensing Tivin Prism Light. Fig. 52 shows part of the Lamlash light in the Firth of ^.- -Q/^* Clyde. Its action will " ^ /->. be easily understood by ^ the numbers shown on ^^7 the diagram. fixed Condensing Lights for more than one Sector of Unequal Range. Fig. 53 is a chart of the Sound of Glenelg, in which the Isle Oronsay light has to be seen generally at a distance of 3 or 4 miles across the Sound, but in the direction AB down the sound an angle of 10 has to be seen about 15 miles and fyi Fro. 51. Horizontal Section. Conden ing twi prism. Lights for sec tors of unequi range. - . .;. "** ^ - -X // /- B */// FIG. 52. Horizontal Section. another up the sound between C and D an angle of 10 has to be seen about 7 miles. Fig. 54 shows in plan the apparatus, which was for certain local reasons not constructed according to the FIG. 48. Horizontal Section, plane and spread equally over the 45 in azimuth. The hemisphere of back rays is condensed into the same arc seawards by the dioptric spherical mirror, which returns the rays incident upon it through FIG. 49. Horizontal Section. FIG. 50. Vertical Elevation. the flame, where mingling with the front rays they are finally dis tributed over the arc of 45. The light passing above the spherical mirror is parallelized by a half holophote Hi (fig. 50), and sent 53. formula, but 193 of the spare light which would otherwise have fallen upon the land were allocated down Sleat Sound and 139 down the Sound of Glenelg. That for Sleat Sound was given FIG. 54. Horizontal Section. FIG. 55. Vertical Section. directly by the main apparatus through the arc 8, and that to Glenelg through the arc 0, but these arcs were respectively strengthened by portions of the holophotes B and C, the light from which was condensed respectively over the arc 8 by each of the straight prisms a and over the arc by each of the prisms I. The condensed light at Isle Oronsay, which is of the fourth order, appears to be equal to that of a first order, so that with a
