8 



PROPERTIES OF LENSES. 



The human eje consists of a convex lens, the 

 crystaline lens placed behind the aperture of the iris, 

 and a screen, the retina, behind the crystaline lens, 

 C and E (Fig. 6). 



FIG. 6. 



A real inverted image of objects is formed on the 

 retina, (how they appear erect to us has not yet been 

 shown), and as the crystaline lens can alter its form, 

 so as to bring convergent or divergent rays to a 

 focus on the retina, the eye can percei7e clearly the 

 same object when placed at different distances. 



There is a limit to the accommodation of the eye, 

 in normal eyes eight or ten inches is the shortest 

 distance an object can be clearly perceived, without 

 giving rise to an unpleasant feeling owing to our 

 efforts to increase the curvature of the lens, and to 

 enable it to bring the divergent rays to a focus on 

 the retina, but if a convex lens L Fig. 6 be placed 

 between the object and the eye it assists in bringing 

 the rays to a focus on the retina. 



The size of the retinal image varies with that of 

 the angle AOB Fig. 6. This angle is called the 

 visual angle. 



