_Mrs. B._ And yet you can see the whole of it, through one of the windows of this room. The image of the house on your retina must, therefore, be smaller than that of the window through which you see it. It is your knowledge of the real magnitude of the house which prevents your attending to its apparent size. If you were accustomed to draw from nature, you would be fully aware of this difference. _Emily._ And pray, what is the reason that, when we look up an avenue, the trees not only appear smaller as they are more distant, but seem gradually to approach each other, till they meet in a point? _Mrs. B._ Not only the trees, but the road which separates the two rows, forms a smaller visual angle, in proportion as it is more distant from us; therefore, the width of the road gradually diminishes, as well as the size of the trees, till at length the road apparently terminates in a point, at which the trees seem to meet. _Emily._ I am very glad to understand this, for I have lately begun to learn perspective, which appeared to me a very dry study; but now that I am acquainted with some of the principles on which it is founded, I shall find it much more interesting. _Caroline._ In drawing a view from nature, it seems that we do not copy the real objects, but the image they form on the retina of our eyes? _Mrs. B._ Certainly. In sculpture, we copy nature as she really exists; in painting, we represent her, as she appears to us. We must now conclude the observations that remain to be made, on the angle of vision. If the rays, proceeding from the extremities of an object, with an ordinary degree of illumination, do not enter the eye under an angle of more than two seconds, which is the 1-1800th part of a degree, it is invisible. There are, consequently, two cases in which objects may be invisible; if they are either so small, or so distant, as to form an angle of less than two seconds of a degree. In like manner, if the velocity of a body does not exceed 20 degrees in an hour, its motion is imperceptible. _Caroline._ A very rapid motion may then be imperceptible, provided the distance of the moving body, is sufficiently great. _Mrs. B._ Undoubtedly; for the greater its distance, the smaller will be the angle, under which its motion will appear to the eye. It is for this reason, that the motion of the celestial bodies is invisible, although inconceivably rapid. _Emily._ I am surprised, that so great a velocity as 20 degrees an hour, should be invisible.