We may fairly consider sight as a species of touching, though very different from what we commonly understand by that sense; for in order to exercise the latter we must be near the object, whereas we can touch with the eye as far as the light the object contains will make an impression, or its figure form an angle therein. This angle, when the object is viewed at the greatest distance, is about the 3436th part of its diameter, therefore an object of a foot square is not visible beyond 3436 feet, or a man of five feet high at a greater distance than 17180 feet. But the extent of vision is in some measure influenced by the light which surrounds us, and we should be enabled to see any object in the night at 100 times greater distance than in the day, provided it was equally illuminated; thus, for instance, we can perceive a lighted candle at full two leagues in the night, supposing the diameter of the luminary to be one inch, whereas in the day we should not be able to discern it beyond the proportion of the above ratio; and as this is a circumstance which attends all objects when viewed at those different periods, we may conclude that one principal reason for our not being able to discern things at a greater distance, is the brilliancy of the light which fills up the intermediate space, and so destroys the reflected rays from those still more distant objects. When we are surrounded with strength of light the objects near make a forcible impression on the retina of the eye, and obliterate those far off, which are weak and faint; and, on the contrary, if we view a luminous body in the night, even at a considerable distance, that becomes perfectly visible, while those which are near are scarcely discernible. From these reasons it is, that a man at the bottom of a deep pit can see the stars, or, by employing a long tube in a dark room, may obtain some effects from the telescope in the middle of the day. From this it is evident, that if bodies were furnished with more strength of light they would be visible at greater distances, although the angle was not increased, for a small candle, which burns bright, is seen much farther off than a flambeau that is dim. Of these facts, relative to the influence of light, we have a still stronger proof in the variation between a microscope and telescope, both of them instruments of the same kind, increasing the visible angles of objects, whether they be really minute or rendered so by distance, and yet the latter does not magnify beyond a thousand times, whereas the former will exceed a million, and this difference plainly arises solely from the degree of light, for could the distant object be additionally illuminated, telescopes would have the same effect upon distant objects as microscopes have upon small bodies. But it is only by comparing the size of the angle formed in the retina of the eye, the degree of light which illuminates the adjacent and intermediate objects, and the