II. On a varying cylindrical lens

A cylindrical lens of continuously varying power has long been a desideratum, and one was constructed and described by Professor Stokes, at page 10 of the Report of the British Association for 1849 (Transactions of the Sections). He points out that— “If two piano-cylindrical lenses of equal radius, one concave and the other convex, be fixed, one in the lid and the other in the body of a small round wooden box, with a hole in the top and bottom, so as to be as nearly as possible in contact, the lenses will neutralise each other when the axes of the surfaces are parallel; and by merely turning the lid round an astigmatic lens may be formed, of a power varying continuously from zero to twice the astigmatic power of either lens.”