3 13 SECTION-MECHANICS. 



very important improvements on the fixed light where he employed t\vo 

 totally new instruments. One cf these was called a cylindric refractor. 

 This consisted of the solid, which is generated by the revolution round 

 a vertical axis of the middle section of the great annular lens. 

 This cylindric refractor formed a hoop encircling the light, having 

 lenticular action only in the vertical plane. These refractors, when 

 of a large size, were made in the form of polygons, but in 1836, Mr. 

 Alan Stevenson, Who was the first to introduce the dioptric system 

 into this country, made the central belt of the first order lights truly 

 cylindrical, and adopted inclined instead of vertical joints for the 

 glass sectors by which an obscuration of the light by the brass set- 

 tings at any part of the horizon was rendered impossible." He after- 

 wards applied the same princip'e to the astragals of the outer 

 lanterns of the largest class of lights. The refractor does not in 

 the least degree interfere with the spread of the light by natural 

 divergence all round the horizon, but it parallelizes all the light that 

 falls upon it in a vertical plane. In order to utilize the light, which 

 passed over the cylindric refractor, Fresncl afterwards introduced a 

 totally new instrument, and not only so, but for the first time intro- 

 duced the principle of total reflection into lighthouse illumination. 

 The way in which he did this was by means of a scries of prisms 

 of a triangular section. The rays striking on the first face of the 

 prism were somewhat refracted, then on the second face they were 

 reflected, and passing to the last surface, emerged, after another 

 refraction, parallel to the light coming from the cylindric refractor. 

 Those rings are arranged horizontally round the vertical axis, and, 

 in connection with the refracting hoop, enclose the flame in a complete 

 cage. 



I have now come to the year 1849, when certain improvements were 

 made in Scotland. It is quite obvious even to a superficial observer 

 that the parabolic reflector is a very imperfect instrument, inasmuch as 

 only a portion of the rays are incident on its surface. What 

 becomes of the rays which escape past that surface ? A large cone 

 of rays does escape past the lips of the reflector, and all of that cone 

 is utterly lost. In order to save this light, what is required is to place 

 a lens inside of the reflector (shown in fig. 3) and at such a distance in 

 front of the focus as to give parallel rays, and not only so, but this 



