When I was invited by your Secretary to deliver a lecture before you, I thought of different subjects, and one occurred to me which I deemed might be of some interest to you. It is one about which I have thought a good deal; and I propose to entitle the lecture “On the Principles of the Chemical Correction of Object-glasses.” I shall, however, leave for the present the chemical rays out of consideration, and deal only with the visual rays, because it is easier to explain the portion of the subject which relates to the chemical rays by reference to the visual–since you can more readily picture them to yourselves, and I can more readily refer to them by naming the colours. When once the principles of the mode of proceeding are understood with reference to the visual rays, it is perfectly simple to apply them to the chemical rays. As for the elementary principles of the subject, I shall not dwell upon them, as I presume they are already familiar to you. You well know that Newton discovered the compound nature of white light, which, on passing through a prism, is bent round and decomposed into different parts, producing different impressions of colour. It was already known that, on passing light through different substances, it was not bent round in the same, but in different degrees.