It may be objected to this theory, that the eye generally receives an equal number of rays at all times, and that therefore a great object cannot affect it by the number of rays, more than that variety of objects which the eye must always discern whilst it remains open. But to this I answer, that admitting an equal number of rays, or an equal quantity of luminous particles to strike the eye at all times, yet if these rays frequently vary their nature, now to blue, now to red, and so on, or their manner of termination, as to a number of petty squares, triangles, or the like, at every change, whether of color or shape, the organ has a sort of relaxation or rest; but this relaxation and labor so often interrupted, is by no means productive of ease; neither has it the effect of vigorous and uniform labor. Whoever has remarked the different effects of some strong exercise, and some little piddling action, will understand why a teasing, fretful employment, which at once wearies and weakens the body, should have nothing great; these sorts of impulses, which are rather teasing than painful, by conti............