512 THE SENSE OF SIGHT. 



presbyopic. Myopia is caused by anything, such as undue con- 

 vexity of the lens, which increases the refracting power of the 

 eye, and so causes the image of the object to be formed at a 

 point anterior to the retina : the defect is remedied by the use 

 of concave glasses. Presbyopia, or long-sightedness, is the 

 result of conditions the reverse of the above, and is remedied 

 by the use of convex glasses, which diminish the focal distance 

 of an image formed in the eye. 1 



3. The direction given to the rays by their refraction is regu- 

 lated by that of the central ray, or axis of the cone, towards 

 which the rays are bent. The image of any point of an object is, 

 therefore, as a rule (the exceptions to which need not here be 

 stated), always formed in a line identical with the axis of the 

 cone of light, as in the line of B a, or A b, Fig. 185 : so that the 

 spot where the image of any point will be formed upon the ret- 

 ina may be determined by prolonging the central ray of the 

 cone of light, or that ray which traverses the centre of the 



FIG. 185. 



pupil. Thus A b is the axis or central ray of the cone of light 

 issuing from A ; B a, the central ray of the cone of light issuing 

 from B ; the image of A is formed at b, the image of B at a, in 

 the inverted position ; therefore what in the object was above 

 is in the image below, and vice versd, the right hand part of 

 the object is in the image to the left, the left-hand to the right. 

 If an opening be made in an eye at its superior surface, so that 

 the retina can be seen through the vitreous humor, this re- 

 versed image of any bright object, such as the windows of the 

 room, may be perceived at the bottom of the eye. Or still 

 better, if the eye of any albino animal, such as a white rabbit, 

 in which the coats, from the absence of pigment, are transpar- 

 ent, is dissected clean, and held with the cornea towards a 

 window, a very distinct image of the window completely in- 

 verted is seen depicted on the posterior translucent wall of the 



1 For details on this subject, consult the various treatises on the 

 Physiology and Defects of Vision. 



