3 i6 SECTION MECHANICS. 



on sound now being carried on by Professor Tyndall, also under the 

 auspices of the Trinity House. All that I intend to do is to describe 

 some of the improvements whichhave originated in Scotland since 1822. 



Perhaps I shall consult your comfort as well as my own convenience 

 best by adopting a somewhat historical form. Prior to the year 1822, 

 the best form of lighthouse apparatus consisted of a silver plated 

 parabola. The optical principle of the parabola is perfectly well 

 known. It is simply this: that all rays emanating from the focus and 

 incident on its surface are rendered parallel to each other, and also 

 parallel to the axis of the apparatus. Of course, if the radiant were 

 a mathematical point, the rays which emanated from the reflector 

 would be strictly parallel, but, inasmuch as instead of employing a 

 mathematical point, a bulky flame is used, the rays which proceed 

 exfocally have a certain amount of divergence, and without that, the 

 instrument would be practically useless for lighthouse illumination. 

 Owing to this divergence you are enabled to place reflectors round 

 about a frame, so as practically to light up the whole 360 degrees of the 

 horizon. In like manner, if you had three or four parabolas, each having 

 a flame in its focus, placed with their axes parallel upon a frame which 

 was made to rotate, then whenever the common axis of the parabolas was 

 pointed to a distant observer, that distant observer would receive a power- 

 ful flash of light, and as the frame moved round, and the axis was turned 

 away from his eye, it would gradually die out and produce darkness. 



In the year 1822 a year which will ever be memorable in the his- 

 tory of lighthouse optics, the distinguished philosopher Augustin 

 Fresnel introduced the dioptric system of lighchouses. For this pur- 

 pose he used a piano convex lens of the same form as had been pro- 

 posed, but for burning purposes only, by Buffon in 1748, and in an im- 

 proved form by Condorcet in 1788. A specimen, the finest I ever 

 saw, is in the museum from the workshop of Messrs. Barbier & Fenestre, 

 of Paris. The principle of the lens was simply this, that instead of 

 having a continuously spherical surface, Buffon proposed to cut out the 

 lens at the back so ab to reduce the thickness of the glass, and to save 

 the light lost by absorption in passing through the glass. Condorcet 

 not only did this, but assuming different radii for curvatures of the 

 different faces, was enabled to correct to a large extent the spherical 

 aberration, and not only so, but to make the lenses in separate pieces. 



