APPRECIATION OF DISTANCES. 287 



when the air is dense and clear, soon after the sun has descended behind 

 it ; so that the name is as appropriate in the vicinity as it was when 

 "the discoverer's eye was bent on it from a distance." 



It is obvious, that without the alternation of light and shade we 

 should be unable to judge, by the eye, of the shape of bodies, to 

 distinguish a flat circle from a globe ; or any of the prominences and 

 depressions, that are every where observable. The universe would seesi 

 to be a flat surface, the outlines of which would not even be perceptible ; 

 and the only means of discriminating objects would be by their differ- 

 ence of colour. It is partly by attending to the varying intensity of 

 light and shade, that the painter succeeds in representing the near as 

 well as the distant objects in an extensive landscape : those in the fore- 

 ground are made bold and distinct; whilst the remote prospect is made 

 to become gradually less and less distinct, until it fades away in the 

 distance. This part of his art is called aerial perspective. 



3. Convergence of the axes. When an object is situate at a moderate 

 distance from us, we so direct the eyes, that if the axes were prolonged 

 they would meet at it. This angle, of course, varies inversely as the 

 distance; so that if the axis be turned to a nearer object, the angle 

 will be greater ; if to one more distant, less. By this change in the 

 direction of the axes the mind is capable of judging, to a certain extent, 

 of near distances. A definite muscular effort is required for each par- 

 ticular case; and the difference in the volition necessary to effect it 

 enables the brain to discriminate, precisely in the same manner as it 

 judges of the height of a body, by the muscular action required to carry 

 the axis from one extremity of the object to the other. 1 We have the 

 most satisfactory evidence, that such convergence of the axes is indis- 

 pensable for judging accurately of distance in near vision. If we fix 

 a ring to a thread suspended from a beam, or attach it to a stand, and 

 endeavour, with one eye closed, to pass a hook, fixed to the extremity 

 of a rod four or five feet long, into the ring, we shall find it impracti- 

 cable unless by accident or by touching the ring with the rod. The 

 hook will generally be passed on the far or near side of the ring; but 

 if we use both eyes, we can readily succeed. They, however, whose 

 eyes are of unequal power, cannot succeed with both eyes. This is 

 shown by the difficulty experienced by those who have lost an eye. 

 M. Magendie 2 says it sometimes takes a year, before they can form an 

 accurate judgment of the distance of objects placed near the eye. The 

 author has known one or two interesting examples, in which the power 

 was never regained; notwithstanding every endeavour to train the 

 remaining organ. 



It need scarcely be said, that the convergence of the axes is no guide 

 to us in estimating objects, which are at such a distance, that the axes 

 are nearly parallel, as the sun and moon, or any of the celestial lumi- 

 naries. 



4. Interposition of known objects. Another mode of estimating the 

 magnitude or distance of objects is by a previous knowledge of the 

 magnitude or distance of interposed or neighbouring objects; and if no 



1 Sir C. Bell, in Philos. Transact, for 1833. 5 Precis, &c., i. 88. 



