X. LIGHTHOUSES, ETC. 537 



2196. Model of Fresnel Revolving Apparatus, as made 

 for 8kerryvore Lighthouse in 1843 ; scale, one-fifth of full size. 



The Commissioners of Northern Lighthouses. 



The light is received and collected into eight horizontal beams by the 

 principal lenses the light which would escape above is collected into eight 

 inclined beams by small lenses, and reflected to the horizon by inclined mirrors. 

 The lower part of the light is sent equally to all parts of the horizon by 

 prismatic rings of glass, which act as mirrors. The rings at Skerryvore are 

 the first that were made of the largest or first order size, and were undertaken 

 by M. Soleil, on the proposal of Mr. Alan Stevenson. 



2197. Model of Stevenson's Holophotal Revolving 

 Apparatus ; scale, one-fifth of full size. 



The Commissioners of Northern Lighthouses. 



The central part of this apparatus consists of eight of Fresnel's lenses. 

 The light which passes above and below these lenses is collected into eight 

 horizontal beams by reflecting prisms. These reflecting prisms were substituted 

 for the inclined lenses and mirrors of Fresnel's first-class revolving apparatus 

 by Mr. Thomas Stevenson, and were first used by him at Singapore, in 1849, 

 on a small scale, and adopted on a large scale atNorch llonaldshay, in Orkney, 

 in 1851. 



2198. Stevenson's Dioptric Holophote. 



The Commissioners of Northern Lighthouses. 



This apparatus collects all the light of the lamp into one beam of parallel 

 ?-ays solely by means of glass. The apparatus constituting the front half of 

 he instrument bends the light that falls upon it into a beam of parallel rays, 

 ^vhile the prisms which constitute the back half are so formed as to prevent 

 idiy light from passing through, and to cause every ray to return back to the 

 fli.me, and to be finally transmitted through the front half, so as to increase 

 f o intensity of the emergent beam. A large red ball is fixed on a rod, so as 

 tc be in focus, to illustrate the action of the instrument. To an observer, the 

 f; :nt half of the apparatus will appear full of red light, but in the back half 

 no red is to be seen, though the lower part of the rod which carries the ball, 

 nn being in focus, is distinctty visible. 



2199. Stevenson's Fixed Azimuthal Condensing Light* 



A^ used at the leading lights of the Biver Tay. 



The Commissioners of Northern Lighthouses, 



It. is remarkable from its employing every kind of dioptric apparatus. The 

 wh le of the light coming from the flame is spread equally over a horizontal 

 arc of 45 by means of the following instruments ; viz., Fresnel's fixed-light 

 api aratus and annular lens, and Mr. T. Stevenson's condensing prisms, holo- 

 phr ie, right-angled conoidal prisms, and dioptric spherical mirror, with Mr. 

 J. T. Chance's setting. 



: 200. Model of Stevenson's Apparent Light. 



The Commissioners of Northern Lighthouses. 



A beam of light, projected on the apparatus in the lantern on the beacon 

 frcr,i a lighthouse on the shore, is reflected or refracted in such a manner as 

 to indicate the position of the beacon at night. It was first used at Stornoway, 

 in T Gotland, in 1852. 



