THE IRIS. 575 



accurate. Spherical aberration causes us no inconvenience, as 

 the iris only allows the central rays to pass, upon which it can 

 produce no noticeable influence. 



Another optical defect in our eyes is astigmatism, depending 

 upon some irregularity of the curvature of the cornea, which may 

 be bent more horizontally than vertically, or vice versd. In either 

 of these cases the light in the vertical and horizontal planes will 

 be differently refracted, so that lines drawn in the two directions 

 will require different adjustments to see them distinctly. This 

 may be at once recognized if we gaze with one eye at the centre 

 from which many sharply-defined lines radiate, when only certain 

 ones can be seen distinctly, unless we move the eye or change its 

 accommodation. When the excessive curvature extends evenly 

 over the whole diameter of the cornea, it gives rise to what is 

 called regular astigmatism, and when the unevenness is localized 

 to one spot of the corneal surface it is called irregular astigmatism. 



The astigmatism which may be called physiological is not 

 noticed by the individual, but pathological astigmatism often 

 occurs, and requires cylindrical glasses to correct it. 



Entoptic images are those which depend on the presence of some 

 opacity or difference in density in the transparent media of the 

 eye itself. They look like variously-shaped specks moving over 

 the field of vision. They are only remarkable when we look at 

 an evenly-colored object or through a pin hole in a black card. 

 In using the microscope they often annoy the unpracticed ob- 

 server. 



THE IRIS. 



It has already been mentioned that the motions of the iris alter 

 the size of the pupillary opening through which the rays of light 

 must pass, and while it regulates the amount of light admitted, 

 it always cuts off a large amount of the marginal rays, acting 

 like the diaphragm of an optical instrument. The great import- 

 ance of not allowing the rays which would traverse the margin 

 of the lens to enter the eyeball can be understood after what has 

 been said of spherical aberration. But the iris also moves so as 

 to contract the pupil when the eye is adjusted for near vision, 

 independently of the intensity of the light by which the object 



