MAGNIFICATION. 175 



pears to the eye to "be erect, because the eye or rather 

 the mind judges the parts of an object to be situated 

 in that direction in which the rays coming from them 

 are impressed upon it. Hence, as in fig. 21, the rays 

 impressed upon the retina at the lower part a, appear 

 to come from the upper part of the cross, although 

 they are lowermost in the image, and so on for the 

 other rays. For distinct vision the rays of each cone 

 must be parallel, or nearly so. 



Angle of Vision. The marginal rays coming from 

 the object cross at a point corresponding to the centre 

 of the pupil, and thus form an angle, as seen in fig. 22, 

 where the cones are omitted, to avoid confusion ; this 

 angle is the angle of vision. Now the size which ob- 

 jects appear to possess is measured by this angle, or 

 by the linear magnitude of their images (i. e. their 

 size estimated in one direction, as of length or breadth) 

 upon the retina. When the object is distant, the an- 

 gle and its linear magnitude are small, and it appears 

 small and distant ; whilst if it be large, or if small 

 and brought near the eye, the reverse will be the case. 



Magnification. Hence an object may be made to 

 appear larger, or may be magnified, by increasing the 

 linear magnitude of its image upon the retina, which 

 can be done by bringing it nearer the eye, as shown 

 in fig. 22, where the image of b formed at b r is larger 

 than the image of a formed at a 1 . But when an object 

 is brought nearer the eye than about 8 or 10 inches 

 (for the distance varies with different persons), its 

 image becomes indistinct and misty ; and this because 

 the rays composing the cones are too divergent to 

 meet at a focus upon the retina, as shown in fig. 23. 

 By interposing a convex lens, however, between the 

 eye and the object, the too divergent rays may be 

 made to meet at a focus upon the retina, as in fig. 24, 

 the object at the same time being rendered apparently 

 larger or magnified, from the refracting action of the 

 lens upon the cones. 



