"The range of the eye, or diameter of the field of vision, is 110°; consequently this is the largest angle under which an object can be seen. The range of vision is from 110° to 1°. . . . The smallest angle under which an object can be seen is upon an average, for different sights, the sixtieth part of a degree, or one minute in space; so that when an object is removed from the eye 3000 times its own diameter, it will only just be distinguishable; consequently the greatest distance at which we can behold an object like a shilling of an inch in diameter, is 3000 inches or 250 feet."

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**The error in perspective, which is almost universally committed, consists in causing lines dissimilarly distant from the eye-line to converge to one and the same vanishing point**. Whereas it is demonstrable that lines most distant from an eye-line must of necessity converge less rapidly, and must be carried further over the eye-line before they meet it at the angle one minute, which constitutes the vanishing point.

A very good illustration of the difference is given in fig. 76. False or prevailing perspective would bring the lines A, B, and C, D, to the same point H; but the true or natural perspective brings the line A, B, to the point W, because there and there only does A, W, E, become the same angle as C, H, E. It must be the same angle or it is not the vanishing point.

'H' is hidden in the field, going from C to W

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The line A, B, represents the altitude of the mast head; E, H, of the observer, and C, D, of the horizontal surface of the sea. By the law of perspective the surface of the water appears to ascend towards the eye-line, meeting it at the point H, which is the horizon. The ship appears to ascend the inclined plane C, H, the hull gradually becoming less until on arriving at the horizon H it is apparently so small that its vertical depth subtends an angle, at the eye of the observer, of less than one minute of a degree, and it is therefore invisible; whilst the angle subtended by the space between the mast-head and the surface of the water is considerably more than one minute, and therefore although the hull has disappeared in the horizon as the vanishing point, the mast-head is still visible above the horizon. But the vessel continuing to sail, the mast-head gradually descends in the direction of the line A, W, until at length it forms the same angle of one minute at the eye of the observer, and then becomes invisible.