I should not have thought of appearing before you to-night to lecture on so hackneyed a subject, had I not been assured by several members of the Council that such an address was really desired by many Fellows of the Society. It is a subject to which I have not paid very special attention, partly because it is in so many and such good hands, and partly because (except from the point of view of theory) it requires for its extension, especially to astronomy, very costly instrumental appliances and a great sacrifice of time.