Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens.

We give a cut of the quern grinding-mill, which, towards the end of the last century, was in use in that country, and which is thus described by Dr. Johnson in his "Journey to the Hebrides:""It consists of two stones about a foot and half in diameter; the lower is a little convex, to which the concavity of the upper must be fitted.

At last he himself constructed an instrument,a leaden organ pipe with two spectacle glasses, both plain on one side, while one of them had its opposite side convex, and the other its second side concave.