Obituary Notices. 
Ixxi 
Fresnel determined the angle subtending the reflecting side of the 
lighthouse prism by the condition that the paths of the extreme 
rays of the intromitted pencil of light should he respectively parallel 
to the sides containing that angle. This construction, although 
elegant, is not essential to the action of the prism, and is even 
disadvantageous, as causing the loss of excentric rays, so that the 
angle of the prism might with advantage he diminished to the 
needed amount. But another disadvantage of Fresnel’s construction 
is this, that while with prisms so devised by him deviations of 
light cannot be obtained exceeding from about 100° to 120° accord- 
ing to the kind of glass employed, by freeing ourselves from the 
limitation which his construction imposes we can quite advantage- 
ously obtain deviations up to 120° or 135°. The utility of this in 
the construction of apparatus to deal with what has been termed 
“the back light” is obvious ; and, accordingly, Mr Stevenson ^tates 
that the late Mr Alan Brebner and himself had designed what they 
termed “back prisms,” by which rays may be made to deviate from 
their original direction by about 130°; and he adds, “I communi- 
cated the description of these prisms to the Boyal Scottish Society 
of Arts on 6th December 1867.* Professor Swan of St Andrews 
also independently proposed the same form of prism, a description 
of which he communicated to the same Society on the 9th Decem- 
ber 1867, accompanied by general formulae for its construction.”! 
These prisms were first used at Lochindal lighthouse in Islay, and 
were made by Messrs Chance in accordance with Professor Swan’s 
formulae. 
The next of Mr Stevenson’s new agents, devised by him in carry- 
ing out his azimuthal condenser system of lighthouse illumination, 
which falls to be described is his Differential Lens. The action of 
this instrument may best be understood by considering first that of 
a Fresnel’s Polyzonal Lens with the source of illumination in its 
principal focus. The light diverging from the flame and falling on 
the plane surface of the lens will, after refraction, be emitted in a 
beam of parallel, and, it may be assumed, horizontal rays. But if 
now for the plane face of the lens be substituted a cylindric surface 
* Trans. Roy. Scot. Soc. Arts, vol. vii., 1868, pp. 540-546. 
t This modification was independently suggested by Mr Chance and by 
myself. 
