Ixx Proceedings of Royal Society of Edinl>im''gh. 
must suffice also merely to name his “Improved Dioptric Holophote 
of 1864 before passing to the consideration of his “Azimuthal 
Condensing System of Lighthouse Illumination.” This, which 
may be regarded as his crowning achievement in improving on the 
former condition of lighthouse optical engineering, occupied his 
attention for a period of fully thirty years (say from 1855 to 1885), 
during which time he expended on it quite a wealth of inventive 
faculty, and in the end may be said to have brought about a total 
revolution in lighthouse construction. 
He himself had not exceeded the literal truth when he wrote 
that “ previous to 1855 lighthouse apparatus, having the same illu- 
minating power in every azimuth, was used not only at places where 
the distances from which the light could be seen were everywhere 
equal, and where the employment of such apparatus was therefore 
quite legitimate, but also for places having a searange much 
greater in some directions than others. This indiscriminate appli- 
cation of apparatus of equal power to the illumination of our coasts 
necessarily involved a violation of economic principle, for the light 
was either too weak in one direction or else unnecessarily strong in 
another.” “ In other cases, where perhaps only half the horizon 
had to be lighted, a single flame in the focus of a fixed apparatus 
could also be strengthened by a hemispheric reflector placed on the 
side next the land.” “ But no attempt was ever made to allocate 
this auxiliary light in proportion to the varying lengths of the 
different ranges and the amplitudes of the arcs to be illuminated ; 
nor, where a light had to show all round the horizon, to weaken its 
intensity in one arc, and with the rays so abstracted to strengthen 
some other arc, which, from its range being longer, required to be of 
greater power. As none of the agents or combinations which we 
have as yet described were sufficient for dealing with this branch of 
lighthouse optics, I found it necessary to devise eight new agents, 
possessing special optical properties, for distributing the rays not 
egually but equitably ” {Lighthouse Construction and Illumination^ 
1881, pp. 97-8). 
Some, at least, of these new agents will now fall to be described, 
and of these the first, taken in the order in which Mr Stevensou 
has described them, is that which he has termed a “ back prism ” (op. 
eit., p. 91). 
