« ForrigeFortsett »
instance at Yorktown from whence the water prospect eastwardly is without termination; wherein a canoe with three men, at a great distance, was taken for a ship with its three masts, I am little acquainted with the phenomenon as it shows itself at sea; but at Monticello it is familiar. There is a solitary mountain about forty miles off in the South, whose natural shape as presented to view, is a regular cone; but by the effect of looming, it sometimes subsides almost totally in the horizon; sometimes it rises more acute and more elevated; sometimes it is hemispherical; aud sometimes its sides are perpendicular, its top flat, and as broad as its base. In short, it assumes at times the most whimsical shapes, and all these, perhaps, successively in the same morning. The Blue Ridge of mountains comes into view, in the North-east, at about one hundred miles distance, and approaching in a direct line, passes by within twenty miles, and goes off to the South-west. This phenomenon begins to show itself on these mountains, at about fifty miles distance, and continues beyond that as far as they are seen.
I remark no particular state, either in the weight, moisture or heat of the atmosphere, necessary to produce this. The only constant circumstances are its appearance in the morning only, and on objects at least forty or fifty miles distant. In this latter circumstance, if not in both, it differs from the looming on the water. Refraction will not account for the metamorphosis. That only changes the proportion of length and breadth, base and altitude, preserving the general outlines. Thus it may make a circle appear eliptical, raise or depress a cone, but by none of its laws, as yet developed, will it make a circle appear a square, or a cone a sphere.” With all due respect and deference to the opinion of this distinguished man, distinguished alike in the history of his country's politicks and her literature, I apprehend, that the phenomena here stated of the canoe, which in the bay of Yorktown is sometimes mistaken for a ship, and of the various appearances exhibited by the mountain to be seen from Monticello, are to be ranged under very different classes, and solved upon different principles. When a canoe, with three rowers in it, is seen at a distance through the mist or towards the dusk of evening, and is mistaken for a ship, this fact is to be classed under those fallacies of vision, which the seaman, indeed, denominates looming, and which although its name is unknown to the systems of philosophy, yet the thing itself is perfectly familiar, and has been repeatedly explained. Nothing is more deceptive than the view of distances upon water, and when to this circumstance is added a misty air and obscurity of the object, another sign of great distance, the smallest things may easily appear to the sight to be the largest. There is nothing more commonly known in optical science than that inversion of the ordinary laws of vision, by which distant objects are made to appear larger than they really are. Every day's rising and setting sun furnishes an example of this kind. As to the singular variety of appearances exhibited by the mountain in the vi. cinity of Monticello, I presume they are all referable to the ordinary laws of the reflection and refraction of light. Such a changeable medium as the mist of the morning which lingers upon a valley below a mountain, would very naturally, when that mountain was beheld through means of the light that passed through it, be altered into the most various and fantastick forms. The writings upon opticks supply us with instances of this kind in the greatest abundance.
I shall conclude this brief article upon deceptions of sight, by an attempt to explain that phenomenon so often observed, and of which so many solutions have been given, the increased dimensions of the sun and moon in their apparent diameter, at their rising and setting. Dr. Smith, in his optics, gives the same solution of this phenomenon, as had before been given by Mallebranche, Dr. Wallis, and others. He remarks that the apparent sky above us, in which the sun
and moon seem to move, is not a complete hemisphere, in the centre of which is placed the eye of the spectator, but a less portion of a spherical surface than a hemisphere, where the eye of the spectator is greatly above the centre of the concavity. Of this flatted concavity of the sky above us, we are all sensible; that is to say, that part of the sky which rests upon the horizon, appears to be much more remote from
than that which is in the meridian over our heads. This, we are told, affords a solution of the phenomenon of the increased size of the rising and setting sun.
For we judge not of the magnitude of any object, says Dr. Wallis, as quoted by Bishop Berkeley, by the visual angle alone, but by the visual angle in conjunction with the distance. Hence, though the angle remain the same, or even become less, yet if withal the distance seem to have been increased, the object shall appear greater. Now one way, whereby we estimate the distance of any thing, is the number and extent of the intermediate objects. When, therefore, the moon is seen in the horizon, the variety of fields, houses, &c. together with the large prospect of the wide extended land or sea, that lies between the eye and the utmost limb of the horizon, suggest unto the mind the idea of greater distance. And this is the true account of the extraordinary largeness, attributed by the mind to the horizontal moon, at a time when the angle subtended by its dis ameter, is not one jot greater than it used to be.
The account here given of this phenomenon, is the same as that of Dr. Smith. I have been the more particular, says the Dr., in considering the apparent figure of the sky, because I do not find it has ever been determined, although it be absolutely necessary to a satisfactory solution of several noted appearances in the heavens; for instance, supposing the arch ABC, (See the Plate.) to represent that apparent concavity; I find the diameter of the sun or moon, will seem to be greater in the horizon than at any proposed alti
tude, measured by the angle AOB, in the proportion of its apparent distances, OA, OB. The numbers that express these proportions, are represented to the eye in the figure over against the corresponding altitudes of the sun or moon, in which the suns or moons placed in the quadrantal arch FG, described about the centre o, are all equal to each other, and represent the body of the moon at the heights there noted; and the unequal moons in the concavity ABC, are terminated by the visual rays that come from the circumference of the real moon, at those heights to the eye at 0. The diameters of these unequal moons, at A and B, do therefore, bear the same proportion to each other as their apparent distances OA, OB; and they must appear in the very same proportion that they really have in this concave, because we judge all objects in the heavens to be in this very surface; and so the appearance to the eye is exactly the same, as if several moons were painted upon a real surface ABC, in the proportions here assigned; though the visible magnitudes of them all answering to their equal images upon the retina, were exactly equal.
This theory is ingenious, and I doubt not, contains in it some degree of truth. To the extent, however, to which it is carried by Dr. Smith, it is justly liable to some objections. There are two objections against it, considered as the sole cause of the phenomenon in question, which at once strike the mind, and cannot easily be obviated.
The one is, that in this calculation there is no allowance made for the different sizes of the sun's apparent disk, as it descends at different times, upon the limb of the horizon; and the other, that if we look at the sun or moon, at its rising or setting, through a window, in such a manner that all prospect of the concavity of the sky, and interposing space of earth, shall be cut off, the same appearance of increased magnitude is exhibited. The first of these objections has