and bright rays by the atmosphere. When such distinction
has been employed, as in the Wolf Rock apparatus, the red and
white beams can be approximately equalized in initial intensity by
constructing the lens and prism panels for the red light of larger
angle than those for the white beams. Owing to the absorption by
the red colouring, the power of a red beam is only 40% of the
intensity of the corresponding white light. The corresponding
intensity of green light is 25%. When red or green sectors are
employed they should invariably be reinforced by mirrors, azimuthal
condensing prisms, or other means to raise the coloured beam to
approximately the same intensity as the white light. With the
introduction of group-flashing characteristics the necessity for using
colour as a means of distinction disappeared.
.jpg) |
| Fig. 35.—Fixed Apparatus at Chassiron Lighthouse (1827). |
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| Fig. 36.—Vertical Section. Prism of Dioptric Spherical Mirror. |
High-Angle Vertical Lenses.—Messrs Chance of Birmingham have manufactured lenses having 97° of vertical amplitude, but this result was only attained by using dense flint glass of high refractive index for the upper and lower elements. It is doubtful, however, whether the use of refracting elements for a greater angle than 80° vertically is attended by any material corresponding advantage.
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| Fig. 37.—Chance’s Dioptric Spherical Mirror. |
Group Flashing Lights.—One of the most useful distinctions consists in the grouping of two or more flashes separated by short intervals of darkness, the group being succeeded by a longer eclipse. Thus two, three or more flashes of, say, half second duration or less follow each other at intervals of about 2 seconds and are succeeded by an eclipse of, say, 10 seconds, the sequence being completed in a period of, say, 15 seconds. In 1874 Dr John Hopkinson introduced the very valuable improvement of dividing the lenses of a dioptric revolving light with the panels of reflecting prisms above and below them, setting them at an angle to produce the group-flashing characteristic. The first apparatus of this type constructed were those now in use at Tampico, Mexico and the Little Basses lighthouse, Ceylon (double flashing). The Casquets apparatus (triple flashing) was installed in 1877. A group-flashing catoptric light had, however, been exhibited from the “Royal Sovereign” light-vessel in 1875. A sectional plan of the quadruple-flashing first order apparatus at Pendeen in Cornwall is shown in fig. 39; and fig. 55 (Plate 1.) illustrates a double flashing first order light at Pachena Point in British Columbia. Hopkinson’s system has been very extensively used, most of the group-flashing lights shown in the accompanying tables, being designed upon the general lines he introduced. A modification of the system consists in grouping two or more lenses together separated by equal angles, and filling the remaining angle in azimuth by a reinforcing mirror or screen. A group-flashing distinction was proposed for gas lights by J. R. Wigham of Dublin, who obtained it in the case of a revolving apparatus by alternately raising and lowering the flame. The first apparatus in which this method was employed was erected at Galley Head, Co. Cork (1878). At this lighthouse 4 of Wigham’s large gas burners with four tiers of first-order revolving lenses, eight in each tier, were adopted. By successive lowering and raising of the gas flame at the focus of each tier of lenses he produced the group-flashing distinction. The light showed, instead of one prolonged flash at intervals of one minute, as would be produced by the apparatus in the absence of a gas occulter, a group of short flashes varying in number between six and seven. The uncertainty, however, in the number of flashes contained in each group is found to be an objection to the arrangement. This device was adopted at other gas-illuminated stations in Ireland at subsequent dates. The quadriform apparatus and gas installation at Galley Head were superseded in 1907 by a first order bi-form apparatus with incandescent oil vapour burner showing five flashes every 20 seconds.
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| Fig. 38.—Gage Roads Direction Light. |
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| Fig. 39.—Pendeen Apparatus. Plan at Focal Plane. |
Flashing Lights indicating Numbers.—Captain F. A. Mahan, late engineer secretary to the United States Lighthouse Board, devised for that service a system of flashing lights to indicate certain numbers. The apparatus installed at Minot’s Ledge lighthouse near Boston Harbour, Massachusetts, has a flash indicating the number 143, thus: - ---- ---, the dashes indicating short flashes. Each group is separated by a longer period of darkness than that between successive members of a group. The flashes in a group indicating a figure are about 112 seconds apart, the groups being 3 seconds apart, an interval of 16 seconds’ darkness occurring between each repetition. Thus the number is repeated every half minute. Two examples of this system were exhibited by the United States Lighthouse Board at the Chicago Exhibition in 1893, viz. the second-order apparatus just mentioned and a similar light of the first order for Cape Charles on the Virginian coast. The lenses are arranged in a somewhat
