"But we must also admit, that we are accustomed to expect in these instruments complete precision of the image in its entire extent, while it is only necessary for the image on the retina to be exact over a very small surface, namely, that of the yellow spot. The diameter of the central pit corresponds in the field of vision to an angular magnitude which can be covered by the nail of one's forefinger when the hand is stretched out as far as possible. In this small part of the field our power of vision is so accurate that it can distinguish the distance between two points, of only _one minute_ angular magnitude, _i.e._ a distance equal to the sixtieth part of the diameter of the finger-nail. This distance corresponds to the width of one of the cones of the retina. All the other parts of the retinal image are seen imperfectly, and the more so the nearer to the limit of the retina they fall. So that the image which we receive by the eye is like a picture, minutely and elaborately finished in the centre, but only roughly sketched in at the borders. But although at each instant we only see a very small part of the field of vision accurately, we see this in combination with what surrounds it, and enough of this outer and larger part of the field, to notice any striking object, and particularly any change that takes place in it. All of this is unattainable in a telescope. "But if the objects are too small, we cannot discern them at all with the greater part of the retina. 'When, lost in boundless blue on high, The lark pours forth his thrilling song,' the 'ethereal minstrel' is lost until we can bring her image to a focus upon the central pit of our retina. Then only are we able to _see_ her. "To _look_ at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision. This we may call _direct_ vision, applying the term _indirect_ to that exercised with the lateral parts of the retina--indeed with all except the yellow spot. "The defects which result from the inexactness of vision and the smaller number of cones in the greater part of the retina are compensated by the rapidity with which we can turn the eye to one point after another of the field of vision, and it is this rapidity of movement which really constitutes the chief advantage of the eye over other optical instruments.... "A great part of the importance of the eye as an organ of expression depends on the same fact; for the movements of the eyeball--its glances--are among the most direct signs of the movement of the attention, of the movements of the mind, of the person who is looking at us." _Popular Lectures on Scientific Subjects_, pp. 212-214.