the other hand, are immediate;
a certain duration of darkness is followed at once and without the least
gradation by a certain period of light. The arrangement employed by my
grandfather to effect this object consisted of two opaque cylindric
shades or extinguishers, one of which descended from the roof, while the
other ascended from below to meet it, at a fixed interval. The light was
thus entirely intercepted.

At a later period, at the harbour light of Troon, Mr. Wilson, C.E.,
produced an intermittent light by the use of gas, which leaves little to
be desired, and which is still in use 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
superiority of this form of intermittent light is economy in the
consumption 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:--

[Illustration: Fig. 1.]

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
deg. of light pass regularly the eye of the seaman; and are followed at
once by 180 deg. 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