660 PHYSIOLOGY 



as a means of judging the size of surrounding objects. Where 

 the size of the object is unknown our judgment of its size is 

 determined by a comparison of its apparent size, as judged 

 from the size of the retinal image, with the muscular effort of the 

 convergence and accommodation which are present at the same time. 

 Thus if we gaze at the sun for a minute so as to gain a negative 

 after-image the size of this image will be constant. Its apparent 

 size, however, will vary according to the distance of the surface on which 

 we direct our gaze : on looking at a piece of paper held near, it may be 

 about one inch across ; on looking at a distant wall, it may be 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 



FIG 300. *ke w ^ re g auze J 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 

 pin-hole 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 contribute to this judgment of 

 size, the convergence of the axes is more important than the accommoda- 

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

 accommodation 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 obj ects no act of accommoda- 

 tion 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 judg- 

 ments 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. 



