Ixiv Proceedings of Boyal Society of Edinhiorgli. 
house system Fresnel’s dioptric apparatus, on the construction of 
which he made some important improvements, among which were 
the employment of diagonal in place of rectangular joints in Fresnel’s 
refractory apparatus, and with the same object — namely, of obtain- 
ing more uniform distribution of light — diagonal astragals in the 
lighthouse lanterns. With a like spirit his successors remodelled 
many of the earlier Scottish lighthouses, replacing the older reflector 
apparatus by the lenses and other agents of the dioptric system ; 
and it was in this work that Thomas achieved the well-earned and 
world-wide reputation of being first in his time in the improvement 
of lighthouse apparatus, “by whose devices,” it has with literal 
truth been said, “ the great sea-lights in every quarter of the world 
now shine more brightly.” 
Augustin Fresnel (b. 1788), in June 1819 placed by the Govern- 
ment of France on the Commission des Phares, in August of the 
same year submitted to the Commission his design of a polyzonal 
lens, which he proposed should supersede the reflectors then in 
use in the lighthouses of France. Owing to the then imperfect 
condition of the art of glass-working, difficulties were at first ex- 
perienced in the manufacture of these lenses, which were only 
overcome by new methods of Fresnel’s own devising ; and it was 
not until July 1823 that the famous old lighthouse tower at 
Cordouan, where it had been determined to inaugurate the new 
system of lighthouse illumination, was lit up by means of lenses 
in place of the reflectors which had served since 1799. 
Meanwhile Fresnel had communicated to the Academy of 
Sciences, on 29th July 1822, his Memoire sur un nouveau systems 
d'eclairage des Phares. This, the only considerable published 
writing on his “ new lighthouse system” proceeding from his own 
pen, consists in a description of his polyzonal lenses and their 
arrangement for revolving lighthouse appai?atus, where, in order 
to save light which would otherwise be lost by upward divergence, 
each large lens is accompanied by a smaller one placed above in 
connection with an inclined mirror, the effect of the arrangement 
being to increase the duration of the flash from the great lens. 
But he died in 1827, and thus but the five remaining years of 
his brief term of life were afforded him in which to work out his 
dioptric system of lighthouse illumination ; but in these years the 
