190 American Quarterly Microscopical Journal. 



Provide a thin brass plate (Fig. 7) perfectly square and flat, 

 of sufficient size to enclose the required parabola, for which it 

 is to serve as a template. In a center line quite parallel with 

 the sides, prick off two equidistances, the directrix (a) with- 

 out and the focus {h) within the vertex of the parabola. Draw 

 a number of parallel lines at right angles across the center line; 

 these lines need not be set at any particular distance, but may 

 be ruled at sight, taking the precaution of setting them close 

 together towards the vertex, and progressively increasing the 

 distance between them towards the base. With fine pointed 

 dividers, take the distances of these lines on the axis from the 

 directrix or outside mark {a) in succession. For each measure- 

 ment shift the point to the focus {h) within the vertex, and bi- 

 sect the line from which the distance was taken on both sides, 

 the intersection of all the lines by the arcs from the focus, will 

 give the outline of a true parabola. The crossing points are 

 now to be dotted in with a thin, sharp pointed center punch, 

 applied at the right spot under a hand magnifier. The surplus 

 brass is cut out with a fine saw, and the template carefully 

 filed up till the punch marks appear as sunk in the metal. 



The block of glass, if intended for a flat-topped or immersion 

 paraboloid, should have both its base and apex polished off to 

 the right thickness, before it is cemented to the lathe-chuck 

 with black sealing wax. By means of the rough edges of aa 

 old saw-file, ground on one side and used with plenty of turpen- 

 tine, the glass is turned away at a very slow speed till it is seen 

 approximately to fit the template. The edges of this are then 

 lightly smeared with reddle and oil, and the paraboloid fine 

 turned with a keen edge, until the template marks it evenly all 

 over. In order to take out the rings left from the turning, a 

 block of brass, not larger than half an inch square, is traversed 

 over the revolving glass with coarse and then smoothing emery, 

 till all scratches disappear. The glass is then polished with a 

 buff stick and crocus and water, and finally, a piece of hard 

 bees-wax is held against it with finer crocus, in order to obtain 

 the last degree of polish. 



If the paraboloid is to be a non-immersion one with a cupped 



op, it may be turned flat on the end, till the required thickness 



is arrived at, and the hemispherical cavity roughly turned out 



to a half-circle template, till the center is brought to the focus;. 



the cavity is then finished in the same way as a concave lens. 



