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
