Such an apparatus using a number of catoptric reflectors is inferior to an ordinary dioptric apparatus, for the following reasons: (1.) The beam is inferior in candle-power unless a great number of reflectors are used. (2.) It is expensive in upkeep, owing to the number of burners which must be employed, one being necessary in each reflector. (3.) The cleaning and trimming of all the burners is a considerable labour. Chance Bros and Co, 1910 Guide\Chance plate6a.jpg
Mirrors of the paraboloid form, for reflecting the rays in a parallel beam, are nearly obsolete for Lighthouse purposes, but Spherical Mirrors are largely used to reflect rays, which would otherwise be lost, back to the source of light, thus strengthening the effect of the latter, through which they pass, and are refracted or reflected with the other rays in the direction required. In small Lights catoptric metallic mirrors are employed. Chance Bros and Co, 1910
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The Catoptric System, is one in which the light is reflected only, the reflector being some highly-polished surface. This system is the most ancient and is now nearly obsolete, except in Light-vessels, where metallic reflectors of paraboloid form are still used, on account of the possible danger of the motion of the vessel breaking glass lenses if that type is employed. However, the modern improvements in. balancing, etc., now render it safe to use lenses in such positions, and it seems likely that the use of the catoptric system for lighting at sea will entirely die out. Chance Bros and Co, 1910 |
The light from a revolving catoptric or reflecting system is much brighter than from a fixed light on either principle, as you have the combined effect of several reflectors, each of which gives an equal amount of light, it is calculated, to 350 or 450 such lights without any reflectors. Findlay, 1862, Ch. III
"It has been generally assumed that the dioptric is preferable to the catoptric system; but while your Commissioners do not controvert this opinion, they have conclusive evidence that many of the catoptric lights in England are not only excellent in themselves, but exceed in efficiency the dioptric lights on its shores. The first part of Question 7, of Circular VIII., addressed to mariners, runs thus :-" What British light have you usually seen farthest off?" And out of the 579 witnesses who have answered this question, the greatest distances are mentioned with reference to the lights at Lundy Island, the Calf of Man, Tuskar, Flamborough Head, Beachy Head, and Cromer; and the greatest numbers of witnesses mention Flamborough Head, the Lizard, Lundy, Beachy Head, the Start, and the South Stack, all of which are catoptric revolving lights, with the exception of the Lizard, which is catoptric fixed, and the Lundy and Start, which are dioptric revolving." Findlay, 1862, Ch. III
1849There is some waste of light in both the systems. In the catoptric it is that angle comprised between the angle formed by the lips of the reflector and the flame and the horizontal ray which strikes the outer edge of the reflector. It is the angle rPa, in the upper part of the diagram. That portion of the light which passes upwards is, of course, lost for useful effect; the other portions maybe considered as serviceable. In the year 1849, Mr. Thomas Stevenson, son of Robert, brother of Alan Stevenson, proposed some arrangements which obviate this loss, upon what is termed the holophotal system. Findlay, 1862, Ch. III
1860In the early apparatus, the upper portion consisted of a series of
catoptric zones, formed of separate pieces of silvered concave glass, arranged
in such a manner as to reflect horizontally the beams thrown on to them. The
degree of curvature and inclination to the plane of the system was determined,
as in the case of the parabolic reflector, by considering their section to be a
portion of such parabolas as would, if carried around the focus, form perfect
reflectors, as will be readily understood by the subjoined Diagram, where the
dotted lines show the form of that portion of the parabola not comprised in the
catoptric zone. The same applies to the lower portion of the system. Findlay,
1862, Ch. III Guide\Findlay