Ixxiv Proceedings of Royal Society of Edinburgh. 
found to mention but one or two of these. The first I will name is 
that which he has termed “ The Apparent Light.” This was first 
designed by him for Stornoway harbour, and erected in 1851. This 
harbour has a very narrow entrance, whose available width is still 
further reduced by the presence of a submerged reef. To build a. 
lighthouse on this reef would have been a very costly undertaking,, 
but Mr Stevenson contrived to light up the hidden danger in 
another way, by availing himself of the already existing lighthouse 
on Arnish Point. In a window near the bottom of its tower 
he placed a lens capable of transmitting a horizontal beam of 
parallel rays to a lantern carried on the top of an iron beacon 
25 feet high, which he built on the submerged reef. There the 
rays are received on a system of vertical prisms, which disperse 
them seaward over an angle of 62°. It is scarcely necessary to* 
add that the light thus dispersed will appear to the mariner as 
if it proceeded from an actual lighthouse built on the submerged 
rock, and not from a lamp placed on the distant shore. 
To this enumeration of Mr Stevenson’s very varied achievements 
in lighthouse illumination, I will add the success which crowned his 
experiments on illuminating beacon or buoys by means of electricity 
conveyed from land through submarine cables, as fully realised 
in the lighting of Gedney’s Channel, leading to New York Harbour. 
Mr Stevenson was a Member of Council of the Scottish Meteoro- 
logical Society from the time the Society was established in 1855, 
and its Honorary Secretary from 1871 to his death in 1887. 
During this long period he not only took an active and earnest 
part in the management of the Society’s affairs, but also made 
original and permanent contributions to the science of Meteorology. 
Of these contributions the most important are these : — 
1. The Stevenson Screen for the protection of Thermometers, 
designed by him in 1864. The object sought to be obtained was 
UNIFORMITY among temperature observations, and in this he suc- 
ceeded so largely that the Stevenson Screen quickly came into use, 
and continues to be used extensively in all parts of the world. 
2. The introduction into meteorological investigations of the 
term “Barometric Gradient” in 1867, which he first applied, and 
with great success, in a discussion of the facts of our great Edin- 
burgh hurricane of January 1868. 
