In Fig. 1, AAA represents in plan an ordinary Fresnel’s dioptric fixed light apparatus, and BB’ a hemispherical mirror (either metallic or dioptric on my father’s principle) which is made to revolve with uniform speed about the burner. This mirror, it is obvious, intercepts the rays of one hemisphere, and, returning them through the flame (less loss by absorption, etc.), spreads them equally over the other. In this way 180° of light pass regularly the eye of the seaman; and are followed at once by 180° of darkness. As the hemispherical mirror begins to open, the observer receives the full light, since the whole lit hemisphere is illuminated with strict equality; and as it closes again, he passes into darkness.
Other characteristics can be produced by different modifications of the above. In Fig. 2 the original hemispherical mirror is shown broken up into three different sectors, BB´, CC´, and DD´; so that with the same velocity of revolution the periods of light and darkness will be produced in quicker succession. In this figure (Fig. 2) the three sectors have been shown as subtending equal angles, but if one of them were increased in size and the other two diminished (as in Fig. 3), we should have one long steady illumination and two short flashes at each revolution. Again, the number of sectors may be increased; and by varying both their number and their relative size, a number of additional characteristics are attainable.
Fig. 2.
Colour may also be introduced as a means of distinction. Coloured glass may be set in the alternate spaces; but it is necessary to remark that these coloured sectors will be inferior in power to those which remain white. This objection is, however, obviated to a large extent (especially where the dioptric spherical mirror is used) by such an arrangement as is shown in Fig. 4; where the two sectors, WW, are left unassisted, while the two with the red screens are reinforced respectively by the two sectors of mirror, MM.
Fig. 3.
Fig. 4.
Another mode of holophotally producing the intermittent light has been suggested by my father, and is shown in Fig. 5. It consists of alternate and opposite sectors of dioptric spherical mirror, MM, and of Fresnel’s fixed light apparatus, AA. By the revolution of this composite frame about the burner, the same immediate alternation of light and darkness is produced, the first when the front of the fixed panel, and the second when the back of the mirror, is presented to the eye of the sailor.