Obituary Notices. 
Ixix 
But it is proper here to mention that zones of this description are 
said to have been constructed for Augustin Fresnel so long ago as 
1826. Yet it would seem that no drawing or description of them 
was ever published, nor were they ever used in any lighthouse. 
If they had existed at that early date, there is evidence to prove 
that they had been forgotten ; nor can there be any doubt that they 
were independently invented by Mr Stevenson. 
And no'w that he had obtained for himself this new and powerful 
auxiliary, he forthwith proceeded to apply it to the improvement of 
Fresnel’s revolving-light apparatus. This had hitherto consisted 
in a system of his great annular lenses surrounding the central 
burner, the light which by upward divergence escaped from their 
action, and would have been lost, being received on a combination 
of smaller inclined lenses, which transmit it to plane mirrors over- 
head, and these finally reflect it outwards towards the horizon, to 
strengthen the light emitted by the great lenses. Now Mr Steven- 
son discarded the complex arrangement of the smaller lenses and 
plane mirrors, replacing it simply by a system of his new holophotal 
prisms. This capital improvement was first introduced in North 
Eonaldshay Lighthouse in Orkney, for which the new prisms were 
made in 1851 by M. Letourneau of Paris. Unquestionably it 
efiected a great saving of light. Fresnel himself, in his Memoir e of 
1822, had estimated that the loss of the light subjected to the suc- 
cessive action of his lenses and plane mirrors amounted to one-half ; 
while Mr J. T. Chance remarks that in Fresnel’s revolving appa- 
ratus, as the focal distance of the accessory lenses is less than one- 
half of the shortest focal distance in the system of reflecting zones, 
the intensity of the light issuing from the former w^ould be scarcely 
more than one-fourth of that transmitted by the latter ; and, in 
addition to this cause of inferiority, is the loss arising at the mirrors, 
so that, on the whole, the modern plan (holophotal) must give light 
fim or six times more intense than that of the former (Fresnel’s) 
arrangement. 
It must suffice here simply to mention that, just as in the case of 
“Fresnel’s revolving light,” so also in his “fixed light varied 
by flashes,” similar improvements were effected by Mr Stevenson 
by substituting holophotal prisms in place of the two-fold refracting 
and reflecting agents employed in the original apparatus ; and it 
