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 from me than it really is: my faculty of distinguishing distance in this case not being sufficient to overcome the prejudice arising from the greater faintness, and other imperfections of the image."

Speaking of the third means of judging of distance, he says: "An extent of ground lying before us, is itself properly an object, the visible length or magnitude of which, is the visible distance of an object placed at the farther end of it. The same observation is also just with respect to the side of a straight wall, or hedge, &c." This is particularly true when the extent of ground or wall, &c. is divided into portions, of the magnitudes of which we can form a tolerably correct estimate: but even this is of use only to a certain extent; for if there were placed before a spectator an unbounded straight line distinctly divided into feet, which would of course assist him extremely in judging of moderate distances, he would find that beyond a certain limit it would be impossible to count the divisions, and there could be collected but a very vague notion of their number. A rod of 10 feet in length placed on the ground, in a line with the body, at 100 yards distance, subtends but an angle of about 2 minutes of a degree, which is supposed to be the smallest appreciable, so that an object subtending such an angle, is called, by some writers on Optics, the minimum visibile.

To show how uncertain our estimates of distance by the eye often are, Harris proceeds: "It is manifest from observation, as well as from the nature of the thing, that a given extent appears longer, according as it contains a greater number of visible parts; and hence two remote and equal distances may appear very unequal, according to the different circumstances of the intervening parts, and the relative elevation of the spectator." "But without regarding the differences on this account, let the spectator have a given station or stand on even ground, and there will be many circumstances which may vary the apparent or visible distances. Thus a hedge having in it several grown trees, generally looks longer than a clipt hedge on the same extent of ground in the open field." "A river at first looks not so broad, as after we have had a side view of a bridge across it: and indeed a given extent of water does not appear so long as the same extent of land, it being more difficult to distinguish parts in the surface of the one than in that of the other." To this we may add, that in sailing on the