590 



PHYSIOLOGY 



from one another less than the diameter of the pupil. On accommodating now for one 

 needle, the other needle appears double. Thus if the eyes are accommodated for the 

 distant needle F (Fig. 301, II), the image of isr is formed behind the retina, and since 

 only a very narrow bundle of rays can pass through the holes in the card two images 



of the needle are formed on the retina. In the 

 same way, if the eye be accommodated for the 

 near needle the image of F falls in front of 

 the retina, and therefore there will be again 

 two images of it on the retina. In the former 

 case, if the hole /3 be covered with a card, the 

 left-hand image disappears. In the latter case, 

 on covering /3 the right-hand image disappears, 

 showing that the apparent position of the object 

 depends on the relation of its image in the 

 retina to the point of fixation, i.e. to the fovea 

 centralis. 



JUDGMENT OF SIZE. The apparent 

 size of an object is determined in the 

 first place by the magnitude of its image 

 formed on the retina, and therefore by 

 the visual angle which the object sub- 

 tends at the optical centre of the eye, as 

 will be evident from the diagram (Fig. 

 302). The apparent size of any given 

 object varies inversely in proportion to 

 the distance. Thus the size of an image 

 on the retina of an object two inches 

 long at a distance of one foot is equal 

 to the image of an object four inches 

 long at a distance of two feet. An 

 object can be seen if the visual angle 

 subtended by it (the angle AcB in 

 Fig. 302) is not less than sixty seconds. 

 This is equivalent to an image on the fovea centralis about 4^ across, 

 which is about the diameter of a cone. 



The visual angle is, however, by no means the most important factor 

 in our judgment of size. Thus as a man walks away from us his size does not 

 appear to vary, although the visual angle subtended by him on our retina 

 is continually diminishing. Where the size of an object is known to us, as in 

 the instance just mentioned, it is used 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 



FIG. 301 . Diagram to illustrate 

 Scheiner's experiment. 



F, the far needle ; N, the near needle ; 

 a and/3, two pin-holes in a piece of card. 

 The continuous lines indicate the path 

 of the rays for which the eye is accom- 

 modated. 



