192 
After this the entire piece is to be divided into two equal 
parts, by any diameter A B, and out of each of these a spec- 
tacle glass is to be formed as in fig. I. In these x x will be 
the convex portions, forming the lower part of each glass 
when completed ; and v v the concave portions, or upper 
halves of each glass. In fig. II. C D represents a section 
of fig. I. along a line from Cto D. It will be seen that each 
glass when finished consists of a portion of a plano-convex 
and a plano-concave lens ; and that these can of course be 
made of any focal length required for correcting the abnormal 
action of the eye upon the visual rays* 
After the above plan of construction had been described by 
means of a diagram, the optician last referred to was satisfied 
of its practicability. He sent the diagram to the firm who 
manufactured lenses for him (and who had before pronounced 
the making of such glasses impossible), and in a few days 
Mr. Atkinson obtained a pair of glasses of the kind lie wished 
* Glassos each being part of a double convex and a double concave lens 
might, if required, be constructed by similar means. 
