_Note._--The terms here made use of, from being unknown in the olden philosophy, need explanation.--_Axis line_: that line of the medium of space which is as the axis of the eye produced to, and terminated by the external object. _Visual line_, the same. _Visual continuity_; the line which is continuous _angularly_ with the termination of the axis line. From the termination of this _continuous_ line, there may be another angular continuity or _line_, as from mirror to mirror. All lines continuous from the axis line and terminated by _the object_ supposed to be seen, and however irregular, are _lines_ of _vision_: the angular point, _the point of_ (first, second, or third) _continuity_. The reader should make a diagram for each case as he proceeds. * * * * * Within the window-closed room, a lighted candle is supposed to fill the entire space with light radiated from the flame: the perception is named light, and is thus wise excited. When the axis line is terminated by the flame, the pressure on the nervous fluid is lessened to the degree which promotes the sensation of luminousness, which seems to be the physical appearance of the flame itself. Again; when, in the same room, the eye is directed to a mirror the like perception is excited, because the visual line is continuous from the point of continuity, or termination of the axis line, to the flame as before. When the axis line is terminated by a piece of furniture, the point of continuity being imperfect and the visual continuity thence to the flame irregular or indirect, the optic pressure on the brain by the axis line excites the sensation of colour, which is imputed to the object, chair, or table.