LIGHTHOUSE 113 



construction obviously puts it in our power to execute those lenses, to -which I have 

 given tho name of polyzonal lenses, of pure flint glass free from veins ; but it pos- 

 sesses another great advantage, namely, that of enabling us to correct very nearly the 

 spherical aberration by making the foci of each zone coincide.' 



This description will enable the reader to understand the system which has been 

 adopted by Fresnel, and carried out by the French Government and by our own Com- 

 missioners of Lights. 



In the fixed dioptric light of Fresnel, the flame" is placed in the centre of the ap- 

 paratus, and within a cylindric reflector of glass, of a vertical refracting power, the 

 breadth and height of a strip of light emitted by it being dependent upon the size of 

 the flame and the height of the reflector itself ; above and below is placed a series of 

 reflecting prismatic rings or zones for collecting the upper and lower divergent rayj\ 

 which, falling upon the inner side of the zone are refracted, pass through tho second 

 side where they suffer total reflection, and, passing out on the outer side of the zone, 

 are again refracted. The effect of these zones is to lengthen the vertical strip of 

 light, the size of which is dependent upon the breadth of the flame and the height of 

 the apparatus. 



In Fresnel's revolving lighthouse, a large flame is placed in the centre of a revolv- 

 ing frame which carries a number of lenses on a large scale and of various curva- 

 tures for the avoidance of spherical aberration. With the view of collecting tho 

 divergent rays above the flame, an arrangement of lenses and silvered mirrors is 

 placed immediately over it. By this compound arrangement the simply revolving 

 character of the apparatus is destroyed, as, in addition to the revolving flash, a ver- 

 tical and fixed light is at all times seen, added to which a great loss of light must be 

 sustained by the loss of metallic reflectors. In 1851, Messrs. Wilkins and Letour- 

 neau introduced a catadioptric apparatus of great utility, which was thus described 

 by them : 



The first improvement had special reference to the light, and produced a consider- 

 able increase in its power, whilst the simplicity of the optical arrangements was also 

 regarded. It consisted, first, in completely dispensing with the moveable central 

 cylindrical lenses ; secondly, it replaced these by a single revolving cylinder composed 

 of four annular lenses and four lenses of a fixed light introduced between them ; but 

 the number of each varying according to the succession of flashes to be produced in 

 the period of revolution. 



The second improvement consisted in a new method of arranging the revolving 

 parts, experience having shown that the arrangements in use were very faulty. 

 A short time is sufficient for the action of the friction-rollers, revolving on two 

 parallel planes, to produce, by a succession of cuttings, a sufficiently deep groove 

 to destroy the regularity of the rotatory movement. To obviate this great incon- 

 venience the friction-rollers were so placed and fitted, on an iron axis with regulating 

 screws and traversing between two bevelled surfaces, and when an indentation was 

 made in one place they could be adjusted to another part of the plates which is not so 

 worn. 



The third improvement produced the result of an increase of tho power of the flashes 

 in reA*:olving lighthouse apparatus to double what had been obtained hitherto. By 

 moans of lenses of vertical prisms placed in the prolongation of the central annular 

 lenses, the divergent rays emerging from the catadioptric zone were brought into a 

 straight line, and a coincidence of the three lenses obtained. 



The whole of the prisms, lenses, and zones are mounted with strength and sim- 

 plicity, accurately ground and polished to the correct curves according to their re- 

 spective positions, so as to properly develope this beautiful system of Fresuel. The 

 glass of which they are composed should be of the clearest crystal colour, and free 

 from that green hue which so materially reduces the power of the light, and is con- 

 sidered objectionable for apparatus of this kind. The lamp by which the apparatus is 

 to be lighted consists of a concentric burner with four circular wicks attached to a 

 lamp of simple construction, the oil being forced up to the burner by atmospheric 

 pressure only, so that there are no delicate pumps or machinery to become de- 

 ranged. 



Stcvcnwn's Revolving Lighthouse. This apparatus consists of two parts. The prin- 

 cipal part is a right octagonal hollow prism composed of eight large lenses, which 

 throw out a powerful beam of light whenever the axis of a single lens comes in the line 

 between the observer and tho focus. This occurs once in a minute, as the frame which 

 bears the lens revolves in eight minutes on the rollers placed beneath. The subsidiary 

 parts consist of eight pyramidal lenses inclined at an angle of 30 to the horizon, and 

 forming together a hollow truncated cone, which rests above the flame like a cap. 

 Abovo thfse smaller lenses (which can only be seen by looking from below) ara 



VOL. III. I 



