OPTICS 267 



having the least curvature, and the second the direction of the 

 meridian of greatest curvature. The normal eye is slightly astig- 

 matic, the vertical meridian having the greatest curvature. Be- 

 cause of astigmatism, of a number of black lines all crossing at a 

 common point only one is clearly seen; the others, especially the 

 one at right angles to that clearly seen, are less sharply defined. 

 Severe astigmatism causes disturbances in vision which may be 

 corrected by the use of cylindrical glasses. 



Entoptical phenomena. Opacities in the refracting media, 

 found in the normal as well as in the diseased eye (cells and fibres 

 cf the vitreous humor, accumulation of dust or flock of mucus in 

 the cornea, etc.), hinder the passage of light and cast shadows on 

 the retina which are seen as opaque objects in the luminous visual 

 field. They can be seen especially well when a source of light is 

 placed in the first focal point from which parallel rays pass through 

 the vitreous humor. If the eye is moved, the apparent position of 

 the opaque object also moves muscse volitantes. Among these 

 also belong the shadows cast by the blood vessels (see page 270). 

 The capillary stream of the retina can, under certain conditions, 

 also be seen entoptically in the form of moving points. 



III. Function of the iris. The iris has two muscles 



(with smooth fibres) : 



1. The sphincter of the pupil, a circular muscle, by the 

 contraction of which the pupil is contracted, is innervated 

 by the fibres of the oculo-motor which pass through the 

 ciliary ganglia and the short ciliary nerves. 



2. The dilator of the pupil, radial fibres by the contrac- 

 tion of which the pupil is dilated. It is innervated by the 

 sympathetic fibres which also pass through the ciliary 

 ganglia. 



The inncrvation of both muscles is tonic. Section of the 

 oculo-motor causes dilation ; section of the sympathetic, con- 

 traction of the pupil. 



The iris is opaque because of its pigment. It serves : 



1 . As a diaphragm to cut off the peripheral rays and 

 thereby prevent spherical aberration (see page 266). 



2. To regulate the amount of light entering the eye. 

 The more light the eye receives the more contracted the 

 pupil. This change in the width of the pupil follows reflex ly 

 upon changes in the amount of light entering the eye. The 



