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SECTION-MECHANICS. 



metrically quite perfect, it is really not so perfect an instrument as might 

 at first sight be considered, owing to the inevitable loss of light from 

 the metal reflector by which a large portion of the rays is absorbed, 

 and annihilated. What was wanted was to substitute for metallic 

 reflection the principle of total reflection, which Fresnel had already 

 successfully employed in the fixed light. In order to do that it was 

 necessary to surround the lenses by prisms, having the same profile 

 as those of the fixed prisms of Fresnel ; but instead of being generated 

 round a vertical axis, they are generated round a horizontal axis, so 

 as to parallelize the rays not only in one plane as in the fixed light, but 

 in every plane. (Shown in fig. 4). On the table is an example of this 



Figure 4. 



g holophotal prisms, a, B, b prisms of dioptric spherical mirror. 



light. But the apparatus described deals only with one half of the 

 diverging rays. The other half falls upon the dioptric mirror which 

 is composed of prisms which have their inner faces ground spherically 

 to the flame as a centre. Entering those surfaces, normally, the rays 

 suffer no refraction, and pass onwards to the reflecting surfaces of the 

 prisms wherebeing twice totally reflected, they return back again through 

 their first surfaces to the flame. In order to do this it is quite obvious 

 that thetwo backfaces of the prism must beportions of parabolas, having 



