surface, we shall put the centre of the picture out of focus, which leaves us no better off than before. But with a lens we have four surfaces to deal with and can set one against another,

Photograph showing very bad edges, though good in centre. much in the same way that has been already done for colour; a little alteration in one can be accompanied by such a corresponding alteration in another that the edges of the field are improved while the centre is no worse off. This way of setting one thing against another becomes even more easy when we have three or four