The great and benevolent men who existed before us, and devoted their time and contemplations to the interesting science of optics, have fully and clearly demonstrated the laws which regulate the action of light, the cause and effect of luminous phenomena, and the principles upon which vision depends. We have the conclusive and unvarying results of their numberless experiments, performed under every modification of circumstances, to guide us in establishing principles and rules of action, which the studious and practical opticians of our own times have tested, and, ascertaining them to be free from error, now adopt and act upon them. The captious and consequential may complain of this admission, as tending to detract from the importance with which they might otherwise be regarded; but the optician, who deserves that name, is not anxious to array himself in borrowed plumage, nor to appropriate as his own that which rightfully belongs to others. Granted that we act upon settled and incontrovertible philosophical principles, is it not infinitely more gratifying to have the assurance of a correct result, than to be for ever experimentalizing without arriving at a satisfactory conclusion? The skilful surgeon and the talented engineer are guided in their operations by certain fixed and universal laws, yet no one will dispute that to perform the duties of either of those professions requires much application, skill, and expertness. Precisely so with the optician of the present day. He has fixed, universal, and certain data for his operations; and it is upon his intimate knowledge of these, and the careful and judicious application of them, that his success depends.