Obituary Notices. 
Ixv 
optical agents which he had devised — each perfect of its kind — 
were two in number, — his polyzonal lens, in which the centre and 
radius of curvature for each zone are separately computed, so as 
almost entirely to get rid of spherical aberration, and his totally 
reflecting lighthouse prisms. Eut the application of these agents 
to the design of his fixed light apparatus he did not live to see 
actually constructed in all the details in which he had conceived 
them. It was not until further experience in the manufacture of 
lighthouse apparatus had rendered it practicable that his designs 
were for the first time carried out in Scotland by Alan Stevenson, 
and then, as at length constructed after what we cannot doubt was 
Fresnel’s own ideal design, his fixed light apparatus may be regarded 
as a thing perfect of its kind. Similarly, his revolving light 
arrangements, as they left his hands, more especially when viewed 
in the light of subsequent improvements, wear the aspect of being 
but a first attempt ; and, accordingly, Alan Stevenson when con- 
templating the arrangements to be made for his lighthouse of 
Skerryvore, dissatisfied with those of Fresnel, introduced the 
substantial improvement of adding fixed prisms below Fresnel’s 
revolving lenses, these Fresnel-prisms, manufactured in Paris, 
being the first ever constructed for the large dimensions of a 
first-order light. Had Fresnel’s life been prolonged, there is every 
reason to believe that the dioptric system of lighthouse illumi- 
nation would have received further development at his own hand ; 
but now, when that hand was for ever still, the distinction of 
being his chief successor in the work of improving lighthouse optical 
apparatus did not fall to any one among his own countrymen, but 
was reserved for Thomas Stevenson, the subject of this memoir. 
This life’s work, judging from the date of his earliest publication 
on the subject, may be said to have begun in 1849, even before his 
appointment as one of the engineers to the Board of Northern 
Lights, and to have been continued, so long as health permitted, 
up to within a few years of his death in 1887. Always on the out- 
look for improvements, and more especially while under the fresh 
stimulus arising out of the erection of some new lighthouse, his 
singularly active mind was continually suggesting something new. 
In the course of years his inventions, greater and smaller, arising in 
this way became so numerous that it is scarcely possible in a brief 
