322 SECTION MECHANICS. 



ever the lenses and the holophotal reflecting prisms come opposite the 

 eye, you get a powerful flash which dies out as the axis is turned away. 

 You will observe the distinction between this apparatus and the one I 

 formerly described as constructed by Fresnel. He required two agents 

 for the purpose of parallelizing the light, whereas it is done here by a 

 single agent which produces a great saving of light. (Fresnel's revol- 

 ving light is shown in fig. i & 2, and the holophotal in fig. 6). 



Figure 6. 



L lenses,/ the holophotal prisms. 



Hitherto I have restricted myself rigidly to matters which were 

 published, but I must now refer to an apparatus which I saw for the 

 first time last Saturday in the museum down stairs. It is a small 

 apparatus which is represented to have been made in 1825 by M. 

 Tabouret for Augustin Fresnel, and it contains prisms which though 

 not arranged for a revolving but for a fixed light, are of the same 

 forms as those I have described, which were first published in 

 1 849. I know nothing of the history of that apparatus, but I can state 

 positively that no description of it was ever published until 1852, and 

 that holophotal prisms were not introduced into lighthouses until 1850 

 when I introduced them, and not only so, but the fact is that during 

 no less than a quarter of a century after 1825, apparatus of even the 

 smallest size, where no difficulty of construction could possibly arise, 



