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

ON A NEW FQRM OF INTERMITTENT
LIGHT FOR LIGHTHOUSES 1
The necessity for marked characteristics in coast illumination
increases with the number of lights. The late Mr. Robert
Stevenson, my grandfather, contributed two distinctions, which
he called respectively the intermittent and the flashing light.
It is only to the former of these that I have to refer in the
present paper. The intermittent light was first introduced at
Tarbetness in 1830, and is already in use at eight stations on
the coasts of the United Kingdom. As constructed originally,
it was an arrangement by which a fixed light was alternately
eclipsed and revealed. These recurrent occultations and
revelations produce an effect totally different from that of the
revolving light, which comes gradually into its full strength,
and as gradually fades away. The changes in the intermittent,
on the other hand, are immediate ; a certain duration of dark-
ness 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 cylin-
dric 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
1 Read before the Royal Scottish Society of Arts on 27th March l&7lj
and awarded the Society's Silver Medal.
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