Vision and Optical Glasses. 185 



from every visible object, in all directions, some 

 more and some less convergent. When you are 

 near, therefore, you see the extreme points of 

 any object by pencils of rays, which converge or 

 meet in an angle more obtuse than when it is 

 at a greater distance; and as the rays cross each 

 other in the eye, a larger image is of course painted 

 on the retina. Thus, in PL XV. fig. 67, the ob- 

 ject ABC is seen by the eye at D, under the angle 

 APC. and the image upon the retina cba is very 

 large ; but to the eye at E, placed at double the 

 distance, the same object is seen under the angle 

 A/?C, which is only equal to half the angle APC. 

 The image cba, therefore, is only half as large in 

 the eye at E as in the eye at D ; and this will 

 sufficiently explain why objects appear smaller 

 in proportion to their distance from the ej/e. 

 Observe, however, that this proposition will admit 

 of some exceptions, where the judgment corrects 

 the sense. Thus, if a man six feet high (and not 

 far distant from the spectator) is seen under the 

 same angle with a dwarf two feet high (say at 

 the distance of three feet from the spectator), 

 still the dwarf will not appear as tall as the man, 

 because the sense is corrected by the judgment, 

 which makes a comparison of both with sur- 

 rounding objects of known size. These ex- 

 ceptions will, however, in general, only take 

 place with respect to near objects, and those 

 with whose forms we are well acquainted. 



From what has been said of the structure of 



