260 



THE HUMAN BODY 



whole, provided the light does not lap over into adjacent elements. 

 Any beam of light entering the eye forms a cone with its apex at 

 the focus. Near the focus a cross-section of the cone consists of 

 a small circular area, which is larger the further away from the 

 focus it is taken. Such an area is known as a dispersion circle. 

 The convergent beams from the points of an object 18 feet away 

 strike the retina before reaching their exact focus, but the disper- 

 sion circles formed by them are too small to stimulate more than 

 one element; the effect is therefore the same as though an accurate 

 focus had been reached, and objects at this distance are seen clearly. 

 Accommodation. Points on objects nearer than 18 feet send 

 into the eye beams so diverging, and therefore focussing so far 



FIG. 87. Diagram illustrating the surfaces at which light is refracted in the 

 eye. 



behind the retina, that the dispersion circles formed at the retina 

 are too large to stimulate only single elements. Near objects, 

 therefore, would not be seen distinctly, did not some change occur 

 in the eye; since we can see them quite plainly if we choose (unless 

 they be very near indeed), there must exist some means by which 

 the eye is focussed or accommodated for looking at objects at dif- 

 ferent near distances. That some change does occur one can, also, 

 readily prove by observing that we cannot see distinctly, at the 

 same moment, both near and distant objects. For example, stand- 

 ing behind a lace curtain, at a window, we can as we choose look at 

 the threads of the lace or at the houses across the street; but when 



