Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
the most important was the disposition of lenses and mirrors above the optical apparatus for the purpose of collecting and driving back the rays which were sent out vertically from the illuminant, so that they might be mingled with the horizontal beam, thereby reinforcing it. At a later date similar equiangular prisms were placed below the horizontal beam so as to catch the light thrown downwards from the luminous source, the result being that finally none, or very little, of the light emitted by the illuminant was lost, except by absorption in the process of bending the rays into the desired direction.
Fig. 4.—A Twenty-Four Panel Light, which was introduced into Certain French Lighthouses.
In this ingenious manner the circle of light is divided into sections, called “panels,” each of which comprises its bull’s-eye and its group of concentric rings and prisms. The extent of this division varies appreciably, as many as sixteen panels being utilized in some instances. In this direction, however, subdivision can be carried too far. Thus, in some of the French lighthouses no less than twenty-four panels were introduced. The disadvantage is obvious. The total volume of light emitted from the luminous source has to be divided into twenty-four parts, one for each panel. But the fewer the panels, the more light is thrown through each, and the correspondingly greater power of the beam. Thus, in a four-panel light each beam will be six times as powerful as that thrown from a twenty-four panel apparatus of the same type.
Fresnel also introduced the system of revolving the optical apparatus, and by the introduction of suitable devices was able to give the light a flashing characteristic, so that it became possible to provide a means of identifying a light from a distance entirely by the peculiarity of its flash. The French authorities were so impressed with the wonderful improvement produced by Fresnel’s epoch-making invention that it was adopted immediately for all French lights. Great Britain followed suit a few years later, while other countries embraced the system subsequently, so that the Fresnel lens eventually came into universal use.
Fig. 5.—A Four-Panel Light.
The ray thrown through each panel is six times as powerful as the beam thrown through a twenty-four panel apparatus.
But the Frenchman’s ingenious invention has been developed out of recognition. To-day only the fundamental basis is retained. Marked improvements were made by Mr. Alan Stevenson, the famous Scottish lighthouse engineer. In fact, he carried the idea to a far greater degree than Fresnel ever contemplated, and in some instances even anticipated the latter’s subsequent modifications and improvements. This was demonstrated more particularly in the holophotal revolving apparatus, the first example of which he designed for the North Ronaldshay lighthouse in 1850, a similar apparatus being devised some years later by Fresnel. In 1862 another great improvement was made by Mr. J. T. Chance, of the well-known lighthouse engineering firm of Birmingham, which proved so successful that it was incorporated for first and third order apparatuses in the New Zealand lights designed by Messrs. Stevenson in the same year.
Fig. 6.—Single Apparatus in Four Panels.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
The French and British investigators, however, were not having things entirely their own way. The United States played a part in these developments, although they did not enter very successfully into the problem. The first lighthouse at Boston Harbour carried candles until superseded by an ordinary lamp, which was hung in the lantern in much the same way as it might have been suspended behind the window of a private dwelling. An inventor, Mr. Winslow Lewis, who confessed that he knew nothing about lighthouse optics, patented what he called a “magnifying and reflecting lantern” for lighthouse work, which he claimed was a lamp, a reflector, and a magnifier, all in one. It was as crude a device as has ever emanated from an inventive brain, but the designer succeeded in impressing the Government so effectively that they gave him £4,000, or $20,000, for his invention. The reflector was wrought of thin copper with a silvered surface, while the magnifier, the essence of the invention, was what he called a “lens,” but which in reality comprised only a circular transparent mass, 9 inches in diameter, and varying from 2½ to 4 inches in thickness, made of bottle-green glass. The Government considered that it had acquired a valuable invention, and was somewhat dismayed by the blunt opinion of one of its inspectors who held contrary views concerning the magnifier, inasmuch as he reported cynically that its only merit was that it made “a bad light worse.”
Fig. 7.—Double Flashing Apparatus: Two Panels and Mirror.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
Fig. 8.—Double Flashing Apparatus: Two Groups each of Two Panels.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
The inventor did not manifest any antagonism to this criticism, but immediately pointed out the great economy in the consumption of oil that was arising from the use of his idea. Indeed, he prosecuted his claims so successfully that he clinched a profitable bargain to himself with the Government. His apparatus had been fitted to thirty-four lights, and he contracted to maintain them on the basis of receiving one-half of the oil previously consumed by the lamps which his invention superseded. This arrangement was in vogue for five years, when it was renewed, with the difference that on this occasion the Government, concluding that the inventor was making too much out of the transaction, reduced the allowance to one-third. Subsequently the invention received higher commendation from the officials than that advanced by the critical inspector, although it must be pointed out that meanwhile the magnifying bull’s-eye had been abandoned, and a new type of reflector introduced, so that the sole remaining feature of the wonderful invention was the lamp. Even that had been modified. When the Lighthouse Board was established in 1852 it abolished the much-discussed invention, and introduced the Fresnel system, bringing the United States into line with the rest of the world.
Fig. 9.—Triple Flashing Apparatus: Three Panels and Mirror.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
One feature of the subject cannot fail to arrest attention. This is the possibility of producing a variety of combinations by the aid of the lenses to fulfil different requirements. The Fresnel, Stevenson, and Chance developments in the science of lighthouse optics facilitated this work very significantly. Accordingly, to-day a variety of lights, evolved from the variations in the mounting of the lenses, is in vogue. For purposes of identification they have been divided into a number of classifications, and, for the convenience of the navigator, are described as lights of the first order, second order, and so on. Broadly speaking, there are seven main groups, or orders, the rating only applying to dioptric or catadioptric lights, indicating the bending of the luminous rays in the desired direction, either by refraction and reflection through the medium of prisms, or a combination of both. Actually there is a distinction between these two, the true dioptric system referring only to refraction, where the ray is bent in the desired direction by a glass agent, known as a “refracting prism.” In the catadioptric system, on the other hand, both methods are employed, since the prism performs the dual purpose of reflecting and refracting the rays. However, in modern lighthouse parlance both are grouped under the one distinction “dioptric.”
The rating or classification of the lights varies according to the inside radius or focal distance of the lens—in other words, the distance from the centre of the light to the inner surface of the lens. The main groups are as follows:
| Hyperradial, | 1,330 | millimetres |
(52·3 |