VISUAL JUDGMENTS 591 



several feet across. If we look through a piece of coarse wire gauze at a distant 

 window, the meshes of the gauze appear to be in the neighbourhood of the 

 window and extremely large ; on directing the gaze then to an object held just 

 in front of the wire gauze, the mesh will look extremely small. On the other 

 hand, if we cut out the movements of accommodation and convergence by 

 looking at a piece of wire gauze 

 through a minute pinhole in a card, 

 the size of the meshes will increase 

 as the gauze is brought near to the 

 eye. In this case we judge of their 

 size entirely by the visual angle 

 they subtend. 



In the muscular elements which FIG. 302. 



contribute to this judgment of size, 



the convergence of the axes is more important than the accommodation 

 of each eye, so that judgment is but little affected if we paralyse accommoda- 

 tion altogether by dropping atropine into the eye. The visual axes may be 

 regarded as practically parallel for any object at a greater distance than five 

 metres ; for such objects no act of accommodation is necessary. In judging 

 of the size of any object beyond this distance we have only the visual angle 

 to go by, which of course gives by itself no information unless we know the 

 distance of the object. Here the obscuration of the outlines of the object 

 in consequence of the deficient transparency of the atmosphere plays a large 

 part in our judgments and may be upset in either direction by changes in 

 transparency of the atmosphere. Thus when walking on the Downs in foggy 

 weather a gigantic object may be seen looming through the mist, which, on 

 advancing a couple of paces, is seen to be a sheep. On the other hand, in the 

 clear air of the Alps the traveller continually under- estimates the size of 

 distant objects, and takes a mass of rocks of the size of St. Paul's for a 

 traveller wending his way up the snow arete. 



ILLUSIONS OF SIZE 



The distance between two points appears longer if a number of points be 

 interposed between the two. Thus if two equal quadrilateral figures be 



divided one by horizontal 

 and the other by vertical lines 

 the one divided by horizon- 

 tal lines will appear elongated 

 vertically, and that divided by 

 vertical lines elongated hori- 

 FIG 303 zontally (Fig. 303). Appar- 



ently it requires a somewhat 



less effort to pass directly from one point to the other than when the gaze 

 has to follow an interrupted line. The eye muscles probably make a 

 separate effort of movement at each interruption of the line. To these 



