134 



Addendum to the foregoing Paper. 



The only difficulty in making such a ring of glass as above 

 described consists in the polishing, so as to leave it free from lines. 

 The method I suggested, and which was applied to the making of 

 those that have already been constructed, was to apply the side of a 

 tool by means of a slide-rest, placed at the requisite angles, and to 

 keep the tool in constant motion to and fro during the revolution of 

 the glass ring in the lathe, by which means a very tolerable polish 

 has been produced. 



This condenser may be made to give any one inclination to the 

 ray, even to as near 90° as can be made to pass through the glass 

 on which the object is placed. 



Having determined what angle the condenser under consideration 

 is to be constructed for, let the slide-rest be arranged so that the 

 outer cone will be inclined to the perpendicular, half the angle of the 

 required inclination, and the inner cone inclined to the base, double 

 the preceding angle, that is, equal to the angle of the emergent ray. 



Fig. 1*, plate XIV., shows the appearance that would be presented 

 by cutting the glass part of the annular condenser in half. Fig. 2* 

 is a perspective and fig. 3* a sectional view of the condenser in its 

 fitting of brass, a being the glass ring, b the reflecting surface, and c 

 any opaque substance, such as black wax, gutta percha, &c. 



