between succes

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 at Pendeen in Cornwall is shown in fig. 39; and fig. 55 (Plate I.) illustrates a double flashing first order light at Pachena Point in British Columbia. Hopl<inson'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 W 'ffffmfii

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ec Ion. . ' l, ya ! , y FIG. 35.-Fixed Apparatus at Chassiron Lighthouse (1827). ~ g the re d colouring, the power of a red beam is only 40% of the A W /' ';, 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. H igh-A ngle Vertical Lenses.-Messrs Chance of Birmingham have manufactured lenses having 97° of vertical amplitude, but this b result was only

7 attained by using

- dense flint glass of

high refractive

I 'e ""Z(:7@:.

a: I .1 . ...

-;; ..... —if 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.

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 V

- 2 .-..

r' »»-

C

FIG. 36.-Vertical Section. Prism of Dioptric Spherical Mirror.

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Y ~-~»~~-e—wi

g , - A

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= ff".

fffQ§ —';;, .- if 1;

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FIG. 37.-Chance's Dioptric Spherical Mirror. 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 cato tric li ht had, hgweve/ii, been exhibited from 'the “ Royal Sovereign 'plight-g/essel in I .

75 sectiona plan of the quadruple-flashing first order apparatus

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FIG. 38.-Gage Roads Direction Light. 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 biform apparatus with incandescent oil vapour burner showing five flashes every 20 seconds.

Flashing Lighls indicating Numbers.-Captain F .A A. Mahan, late engineer secretary

for that service a

to the United States Lighthouse Board, devised syiem of flaéphing

ig ts to in icate, "W .

certain numbers. ~§ ::L';-

The apparatus in- 7stalled

at Minors / .

Ledge lighthouse

near Boston Har- ff/ &°

b }p u r, M la s s a -, ;f-?'%l§§ f§ %@ff'3'~f-'-'>'-' % c usetts, as a % * fy' ' N

flash indicating;i "°'° g,

the number 143, W E5 i W

the - — 1 /

- -und' ' -~¢z»n» 1»>»¢-»»

tihe dashes in- .L w - ¢;§ k,5, - . icatin short f '72 »°

flashes.g E a c h q§ "9?=f<f¥:§ ?f€#f>'@7"i group is separ-, ated by a longer

period of dark- ness than that

FIG. 39.-Pendeen Apparatus.

Sive members Of Plan at Focal Plane. a group. The

flashes in a group indicating a figure are about 1% 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 18931 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