Port Lights.—Small self-contained lanterns and lights are in common use for marking the entrances to harbours and in other similar positions where neither high power nor long range is requisite. Many such lights are unattended in the sense that they do not require the attention of a keeper for days and even weeks together. These are described in more detail in section 6 of this article. A typical port light consists of a copper or brass lantern containing a lens of the fourth order (250 mm. focal distance) or smaller, and a single wick or 2-wick Argand capillary burner. Duplex burners are also used. The apparatus may exhibit a fixed light or, more usually, an occulting characteristic is produced by the revolution of screens actuated by spring clockwork around the burner. The lantern may be placed at the top of a column, or suspended from the head of a mast. Coal gas and electricity are also used as illuminants for port lights when local supplies are available. The optical apparatus used in connexion with electric light is described below.

”Orders” of Apparatus.—Augustin Fresnel divided the dioptric lenses, designed by him, into “orders” or sizes depending on their local distance. This division is still used, although two additional “orders,” known as “small third order” and “hyper-radial” respectively are in ordinary use. The following table gives the principal dimensions of the several sizes in use:—

Table II.

Order. Focal
Distance,
mm.		 Vertical Angles of Optics.
(Ordinary Dimensions.)
Dioptric
Belt only.		 Holophotal Optics.
Lower
Prisms.		 Lens. Upper
Prisms.
Hyper-Radial 1330  80° 21° 57° 48°
1st order 920 92°, 80°, 58° 21° 57° 48°
2nd order 700 80° 21° 57° 48°
3rd order 500 80° 21° 57° 48°
Small 3rd order 375 80° 21° 57° 48°
4th order 250 80° 21° 57° 48°
5th order  187.5 80° 21° 57° 48°
6th order 150 80° 21° 57° 48°

Lenses of small focal distance are also made for buoy and beacon lights.

Fig. 42.—Cape Naturaliste Apparatus.
Fig. 43.—Île Vierge Apparatus.

Light Intensities.—The powers of lighthouse lights in the British Empire are expressed in terms of standard candles or in “lighthouse units” (one lighthouse unit = 1000 standard candles). In France the unit is the “Carcel” = .952 standard candle. The powers of burners and optical apparatus, then in use in the United Kingdom, were carefully determined by actual photometric measurement in 1892 by a committee consisting of the engineers of the three general lighthouse boards, and the values so obtained are used as the basis for calculating the intensities of all British lights. It was found that the intensities determined by photometric measurement were considerably less than the values given by the theoretical calculations formerly employed. A deduction of 20% was made from the mean experimental results obtained to compensate for loss by absorption in the lantern glass, variations in effects obtained by different men in working the burners and in the illuminating quality of oils, &c. The resulting reduced values are termed “service” intensities.

As has been explained above, the effect of a dioptric apparatus is to condense the light rays, and the measure of this condensation is the ratio between the vertical divergence and the vertical angle of the optic in the case of fixed lights. In flashing lights the ratio of vertical condensation must be multiplied by the ratio between the horizontal divergence and the horizontal angle of the panel. The loss of light by absorption in passing through the glass and by refraction varies from 10% to 15%. For apparatus containing catadioptric elements a larger deduction must be made.

The intensity of the flash emitted from a dioptric apparatus, showing a white light, may be found approximately by the empirical formula I = PCVH/vh, where I = intensity of resultant beam, P = service intensity of flame, V = vertical angle of optic, v = angle of mean vertical divergence, H = horizontal angle of panel, h = angle of mean horizontal divergence, and C = constant varying between .9 and .75 according to the description of apparatus. The factor H/h must be eliminated in the case of fixed lights. Deduction must also be made in the case of coloured lights. It should, however, be pointed out that photometric measurements alone can be relied upon to give accurate values for lighthouse intensities. The values obtained by the use of Allard’s formulae, which were largely used before the necessity for actual photometric measurements came to be appreciated, are considerably in excess of the true intensities.