Ixviii Proceedings of Royal Society of Edinburgh. 
reflecting the “ back ” rays of the flame in his holophotal apparatus 
arrangements by dioptric agency to the exclusion of metallic reflec- 
tion, the next problem which suggested itself to Mr Stevenson was 
how to obtain a dioptric arrangement which should be capable of 
dealing with the whole of the front rays, hitherto in part subjected 
to metallic reflection. A Fresnel polyzonal lens had, indeed, re- 
ceived the central pencil of these rays, diverging all round the 
horizontal axis of the lens through an angle of about 45°, which, 
after refraction, Avere emitted in directions parallel to that axis. 
Beyond some such limit of divergence, varying with the kind of 
glass of which the lens v/as composed, mere lenticular action Avas 
unavailing toAvards emitting a parallel beam of rays. But now it 
occurred to Mr Stevenson that the agency of which he Avas in quest 
Avas to be found in Fresnel’s catadioptric zones, provided they were 
generated by revolution round a horizontal instead of a vertical 
axis ; and thus, by means of a Fresnel polyzonal lens surrounded 
by a series of totally reflecting rings, generated in the manner noAV 
described in front and with a totally reflecting dioptric mirror 
behind, Mr Stevenson obtained a holophote, in which, instead of 
the optical agents being, as in his earlier invention, partly metallic 
reflectors and partly lenticular, all Avere noAV dioptric. 
Zones of glass the same in section with those in Fresnel’s fixed 
light, but generated by the revolution of their section about a 
horizontal instead of a vertical axis, which he had thus devised in 
order to complete his dioptric holophote, he not unnaturally termed 
“ holophotal.” * The two species of zone, although identically the 
same in section yet differing in their mode of generation, conse- 
quently differ in their optical effect. Light from the focal point 
incident on a Fresnel’s fixed-light zone is emitted in every azimuth, 
but all within a horizontal plane. Light similarly incident on the 
so-called “ holophotal ” zone is all emitted in one and the same 
direction parallel to the axis of revolution of the zone, so that its 
action is identical with that of a lens in rendering parallel rays 
proceeding from its principal focus, but capable of producing much 
greater deviations than lie Avithin the power of any lens. Lenti- 
cular action was thus extended from causing deviations from about 
45°— the limit of Fresnel’s polyzonal lens — to about 130°. 
* Lighthouse Construction and Illumination, 1881, pp. 83-85. 
