INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



621 



Witli the slide rest take off tlie section a, remove the cone, and on 

 the jjarabolic face screw a well-flattened piece of sheet brass slightly 

 exceeding it in size ; back this uj) by a block of the same wood as the 

 cone ; fix both thereto by two countersunk screws passing through 

 holes drilled in the brass plate. The cone is now returned to the 

 lathe centres and the surplus piece of wood turned down, together with 

 the edge of the brass plate, by means of the slide rest till the cone is 

 again complete. A dead smooth file may then be held against the 



Fig. 2. 



Fig. 



revolving cone ; this trims the edge of the contained template which 

 comes out as a true parabola. Unless this is made to match a parabolic 

 figure of known focus, it may be necessary to ascertain the focal point 

 of the blank parabola. This can be easily found as follows (Fig. 2) : — 

 Draw a line a equal to the diameter of the base of the parabola ; take 

 a perpendicular to this &, equal to the height from the base to the 



Explanation of Figures. 

 Fig. 1. — A wooden cone clamped down by screws on to the face plate of a 

 lathe. The axis from which the cone was turned inclined in the direction shown. 

 The dotted section a is turned off parallel with the opposite side ; on the para- 

 bolic face the template is formed. 



Fig. 2. — Method of finding the focal distance of a blank parabolic figure. 

 a. Diameter of base. 

 6. Distance from base to vertex. 

 d Half tlie semidiameter. 

 Connect (/ with end of 6 by line c ; a perpendicular from this, taken from d at 

 the point where it intersects the axis below the base, will be equal to the focal 

 distance below the vertex. 



Fig. 3. — Outline of rectangular brass plate to form a template for paraboloid. 

 h. Focal distance. 



a. Equal to focal distance above vertex of parabola. 

 Cross lines drawn at irregular but increasing distances, as shown, measure- 

 ments on the axis, by compasses, from a to each of these lines ; each line bisected 

 by the same measurements from the focus or point 6 describes the outline of a 

 parabola. 



The dotted segment cf a circle is struck from the focus h representing a non- 

 immersion paraboloid. 



VOL. II. 2 T 



