964 JUDGMENT OF DISTANCE. [Book hi. 



parallel, but diverging or converging according to the direction 

 of the cross-lines ; the direction of the cross-lines affects our 

 perception of the distance between the parallel lines. 



§ 600. Judgment of Distance and Actual Size. The size of 

 the retinal image gives us by itself a measure not of the real 

 size but only of the apparent size of the object. The size of 

 the retinal image will depend on the distance of the object and 

 on the dioptric arrangements of the eye; with the same dioptric 

 arrangements it will depend on the angle subtended by the 

 diameter of the object, and this may be the same for a small 

 object near as for a large object far off. In order to form a 

 judgment as to the actual size of an object, we must adjust our 

 perception of the apparent size by means of a judgment of the 

 distance at which the object is placed; and here the great use 

 of two eyes comes in. 



Even with one eye we can, to a certain extent, form a judg- 

 ment not only as to the position of the object in a plane at 

 right angles to our visual axis, but also as to its distance from 

 us along the visual axis. If the object is near to us, we have 

 to accommodate for near vision; if far from us, to relax our 

 accommodation mechanism so that the eye becomes adjusted 

 for distance. The muscular sense of this effort enables us to 

 form a judgment whether the object is far or near. Seeing the 

 narrow range of our accommodation, and the slight muscular 

 effort which it entails, all monocular judgments of distance 

 must be subject to much error. Every one who has tried to 

 thread a needle or to pour out a glass of wine without using 

 both eyes, knows such errors. 



When, on the other hand, we use two eyes, we have still 

 the variations in accommodation, and in addition have all the 

 assistance which arises from the muscular effort of so directing 

 the two eyes on the object that single vision shall result. 

 When the object is near, we converge our visual axes; when 

 distant, we bring them back towards parallelism. This neces- 

 sary contraction of the ocular muscles affords a muscular sense, 

 by the help of which we form a judgment as to the distance of 

 the object. We can judge of the distance of a vertical line 

 more easily than of a horizontal line, because we can converge 

 our vision more easily upon the former; this is seen in attempt- 

 ing a 'high jump' over a horizontal cord, the judgment of the 

 distance of the cord is facilitated by hanging a vertical cord or 

 tape to it. Conversely, when by any means the convergence 

 which is necessary to bring the object into single vision is 

 lessened, the object seems to become more distant; when the 

 convergence is increased, the object seems to move towards us; 

 this may be seen in the stereoscope. 



The judgment of size is, as we said above, closely connected 

 with that of distance. The real size of the object can be inferred 



