160 JUDGMENT OF DISTANCE. [BOOK in. 



appear not 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. 



798. 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 judgment 

 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. Everyone 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 as- 

 sistance 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. Tliis necessary 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 attempting a 'high jump' over a hori- 

 zontal 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 

 from the apparent size, that is to say from the size of the retinal 

 image, only when the distance of the object from the eye is 



