Dioptric Light erected at Kirlccaldy. 251 



generated by turning the section of a common lens round a line 

 passing through the focus, and perpendicular to the axis of the 

 common lens. The focus is thus in the interior of the annular 

 lens, and the rays proceeding from it, instead of being conver- 

 ged to a conjugate focus, are flashed out horizontally. On the 

 large scale, these lenses are built of many zones, but, in the 

 case in hand, only one piece of glass was used ; and the nicety 

 was this, to give to the surface of the small lens that variation 

 of curvature, which is attained on the separate zones of the 

 large lenses. 



Three classes of difficulties presented themselves, first, the 

 detection of the proper curve ; second, the manufacture of the 

 lump of glass ; and third, the cutting and polishing it. 



The first difficulty belongs to applicate geometry ; and the 

 detail of the method of resolving it, would be here somewhat 

 out of place. It may be enough to notice, that, the cylindric 

 form having been determined on for the interior surface, on ac- 

 count of the facilities which it promised in the manipulation, 

 the outline of the exterior surface necessarily became a curve 

 of high order. The accompanying drawing sliews the section 

 of the lens full size ; as it resulted from very laborious calcula- 

 tions. 



The form of the lens being thus obtained, the next business 

 was to procure the glass, and here obstacles presented them- 

 selves much greater than I anticipated. My first idea was to 

 use flint-glass on account of its high refractive power, but after 

 many attempts, of which the most successful result is presented 

 to the Society, I abandoned that idea, and fell back upon 

 crown-glass. The Messrs Cookson of Newcastle, furnished me 

 with two pieces, which reached me entire, but one, and unfor- 

 tunately the better, had received a blow on one of its edges, 

 and a tendency to split shewed itself soon after commencing 

 operations : it also accompanies the paper. The other was per- 

 fectly sound. On account of the lower refractive index of 

 crown-glass, the lens was carried to less height than had it been 

 of flint-glass. 



My first business was to bore out the cylinder. For this 

 purpose, I fixed a tin plate on the point of my drilling spindle, 

 and having primed the edge of it with diamond powder, I cut 



