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 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.