VISUAL JUDGMENTS 



659 



The projection of visual impressions is well shown in Schemer's experiment. 

 Two needles are placed one behind the other on a wooden rod, one at 18 cm. 

 and the other at a distance of 60 cm. from the eye. One eye is closed, and 

 then a card is held before the other. In the card two small holes are pierced 

 by means of a needle at a distance 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. 299, II), the image of N 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 ft 

 be covered with a card, the left-hand 

 image disappears. In the latter case, 

 on covering ft the right-hand image dis- 

 appears, showing that the apparent posi- 

 tion 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 deter- 

 mined 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. 300). The apparent 

 size of any given object varies in- 

 versely in proportion to the distance. 

 Thus the size of an image on the 

 retina of an object two inches long at 



FIG. 299. Diagram to illustrate Schei- 



ner's experiment. 



F, the far needle ; N, the near 

 needle ; a and (3, two pinholes in a 

 piece of card. The continuous lines 

 indicate the path of the rays for which 

 the eye is accommodated. 



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. 300) is not less than sixty seconds. This is equivalent to 

 an image on the fovea centralis about 4/a 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 



