450 SEC. 12. APPLIED MECHANICS. 



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 at North Ronaldshay, in Orkney, 

 in 1851. 



2198. Dioptric Holophote, designed by Mr. T. Stevenson 

 for lighthouse illumination. 



W. and T. Stevenson, Northern Lighthouse Office. 



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

 rays solely by means of glass. The apparatus constituting the front half of 

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

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

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

 flame, and to be finally transmitted through the front half, so as to increase 

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

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

 front half of the apparatus will appear full of red light, but in the back half 

 no red is to be seen, though the wire which carries the ball, not being in focus, 

 is distinctly visible. 



2199. Fixed Azimuthal Condensing Light. Designed 

 for the leading lights of .the River Tay, by Messrs. Stevenson, 

 civil engineers, Edinburgh. 



W. and T. Stevenson, Northern Lighthouse Office. 



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

 whole 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 

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

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

 J. T. Chance's setting. 



2200. Model of an Apparent Light. 



W. and T. Stevenson, Northern Lighthouse Office. 



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

 from 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 Scotland, in 1852. 



2201. Model of the Laxnlash Apparatus, showing the new 

 twin prisms lately described by Mr. Thomas Stevenson in 

 " Nature," which are now for the first time being constructed, and 

 the new back prisms first introduced at Lochindaal Lighthouse, in 

 Islay. W. and T. Stevenson, Northern Lighthouse Office. 



2202. Lighthouse Apparatus. 



1. Lens with echelons. 1st essay. Fresnel, inventor, 1819. 



2. Polygonal lens, of the first order. Do. do. 1820. 



3. Annular lens, of the first order. Do. do. 1821. 



4. Apparatus for fixed lights. Do. do. 1824. 



5. Apparatus with catadioptric rings. Do. do. 1825. 



