114 LIGNITE 



placed eight plane mirrors, -whose surfaces being inclined to the horizon at 50 in the 

 direction opposite to that of the pyramidal lenses, finally cause all the light made 

 parallel by the refraction of these lenses to leave the mirror in a horizontal direction. 

 The only object of this part is to turn to useful account, by prolonging the duration 

 of the flash, that part of the light which would otherwise escape into the atmosphere 

 above the main lenses. This is effected by giving to the upper lenses a slight hori- 

 , zontal divergence from the vertical plane of the principal lenses. Below are five 

 tires of totally reflecting prisms, which intercept the light that passes below the great 

 lenses, and by means of two reflections and an intermediate refraction project them in 

 the shape of a flat ring to the horizon. 



Stevenson's fixed dioptric apparatus of the first order (same as that at the Isle of 

 May, with various improvements). The principal part consists of a cylindric belt of 

 glass which surrounds the flame in the centre, and by its action refracts the light in 

 a vertical direction iipward and downward, so as to be parallel with the focal plane of 

 the system. In this way it throws out a flat ring of light equally intense in every 

 direction. To near observers, this action presents a narrow vertical band of light, 

 depending for its breadth on the extent of the horizontal angle embraced by the eye. 

 This arrangement therefore fulfils all the conditions of a fixed light, and surpasses in 

 effect any arrangement of parabolic reflectors. In order to save the light which would 

 be lost in passing above and below the cylindrical belt, curved mirrors with their 

 common focus in the lamp were formerly used ; but by the present engineer, the 

 adaptation of catadioptric zones to this part of the apparatus was, after much labour, 

 successfully carried out. These zones are triangular, and act by total reflection, the 

 inner face refracting, the second totally reflecting, and tho third or outer face, a second 

 time refracting, so as to cause the light to emerge horizontally. The apparatus has 

 received many smaller changes by the introduction of a new mode of grouping the 

 various parts of the framework, by which tho passage of the light is less obscured in 

 every azimuth. 



Mechanical lamps of four wicks are used in these lighthouses ; in these the oil is 

 kept continually overflowing by means of pumps which raise it from the cistern below ; 

 thus the rapid carbonisation of tho wicks, which would be caused by the great heat, 

 is avoided. The flames of the lamp reach their best effect in three hours after light- 

 ing, i.e. after the whole of the oil in tho cistern, by passing and repassing over the 

 wicks repeatedly, has reached its maximum temperature. After this the lamp often 

 burns fourteen hours without sensible diminution of the light, and then rapidly falls. 

 The height varies from sixteen to twenty times that of the argand flame of an inch in 

 diameter; and the quantity of oil consumed by it is greater nearly in the same 

 proportion. 



In Stevenson's ordinary parabolic reflector, rendered holophotal (where the entire 

 light is parallelised) by a portion of a catadioptric annular lens, the back part of the 

 parabolic conoid is cut off, and a portion of a spherical mirror substituted, so as to 

 send the rays again through the flame ; while his Jiolophotal catadioptric annular 

 lens apparatus is a combination of a hemispherical mirror and a lens having totally- 

 reflecting zones ; the peculiarity of this arrangement is, that the catadioptric zones, 

 instead of transmitting the light in parallel horizontal plates, as in Fresnel's appa- 

 ratus, produces, as it were, an extension of the lenticular or quaquaversal action of tho 

 central lens by assembling the light around its axis in the form of concentric hollow 

 cylinders. 



Mr. Chance, of Birmingham, constructed a lighthouse which may be regarded as 

 Fresnel's revolving light rendered holophotal. This arrangement was divided into 

 three compartments, the upper and lower of which were composed respectively of 

 thirteen and six catadioptric zones which produce the vertical strip of light extending 

 the whole length of the apparatus, and is similar to Frcsnel's dioptric light. The 

 central or catoptric compartment consisted of eight lenses of three feet focal length, 

 each of which was the centre of a series of eleven concentric prismatic rings, designed 

 to produce the same refractive effect as a solid lens of equal size. These compound 

 lenses were mounted upon a revolving frame and transmitted horizontal flashes of 

 light as they successively rotated. The motion was communicated to the frame by 

 a clock movement, and performs one revolution in four minutes; consequently, as 

 there are eight lenses, a flash of light is transmitted every thirty seconds to the 

 horizon. 



XiXGir AXiOES. This wood is supposed to be the produce of Aquilaria Agallocha, 

 one of the Lace-bark order. 



iiiCNKOus MATTER is vegetable fibre. See FIBRE, VEGETABLE. 

 HGHITE. Under BROWN COAL, and COAL, tho characteristics of lignite have 

 already received attention, therefore littlo further need be said. The term lignito 

 should be confined to fossil wood, or, still more correctly to wood which has under- 



, 



