Collected works > Edinburgh edition, 1894-98 - Works of Robert Louis Stevenson > Volume 28, 1898 - Appendix

NEW FORM OF INTERMITTENT LIGHT
at Troon harbour. By a simple mechanical contrivance, the
gas jet was suddenly lowered to the point of extinction, and,
after a set period, as suddenly raised again. The chief superi-
ority of this form of intermittent light is economy in the con-
sumption of the gas. In the original design, of course, the oil
continues uselessly to illuminate the interior of the screens
during the period of occultation.
Mr. Wilson's arrangement has been lately resuscitated by
Mr. Wigham of Dublin, in connection with his new gas-burner.
Gas, however, is inapplicable to many situations ; and it has
occurred to me that the desired result might be effected with
strict economy with oil lights, in the following manner : —
Fig. 1.
In Fig. 1, A A A represents in plan an ordinary FresneFs
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
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