832 THE EYE AND VISION [CH. LVIIL 



When the spot of light moves against the mirror's movements 

 from the first, then the observed eye is myopic, and the myopia is 

 greater than 1 D. The " point of reversal " is determined by intro- 

 ducing concave lenses of increasing strength into the spectacle frame, 

 until the spot moves in the same direction as the mirror, and the 

 spectacles ordered must have the value of the lens which accomplishes 

 the reversal plus 1 D. to allow as before for the normal eye. 



Many people have differences in the refractive error of their two 

 eyes ; so each should be tested separately. If the observed eye is 

 astigmatic, the observations are more complicated, and must be made 

 in the different meridians of the eye, and the point of reversal 

 determined in each meridian by means of suitable cylindrical lenses. 



FUNCTIONS OF THE IRIS 



The iris has the following two uses : 



1. To act as a diaphragm in order to lessen spherical aberration 

 in the manner just described. This is specially necessary when one 

 wishes to obtain a clearly defined image of an object; the pupil 

 therefore contracts when accommodation for a near object takes 

 place. 



2. To regulate the amount of light entering the eye. In a bright 

 light the pupil contracts ; in a dim light it enlarges. This may be 

 perfectly well seen in one's own iris by looking at it in a mirror 

 while one alternately turns a gas-light up and down. 



The muscular fibres (unstriped in mammals, striped in birds) of 

 the iris are arranged circularly around the margin of the pupil, and 

 radiatingly from its margin. The radiating fibres are best seen in 

 the eyes of birds and otters; some look upon them as elastic in 

 nature, but there is little doubt that they are contractile. Those 

 who believe they are not contractile explain dilatation of the pupil 

 as due to inhibition of the circular fibres. But if the iris is stimu- 

 lated near its outer margin at three different points simultaneously 

 the pupil assumes a triangular shape, the angles of the triangle 

 corresponding to the points stimulated ; this must be due to con- 

 traction of three strands of the radiating muscle ; inhibition of the 

 circular fibres would occur equally all round. 



The iris is supplied by three sets of nerve-fibres contained in 

 the ciliary nerves. 



(a) The third nerve via the ciliary ganglion and short ciliary 

 nerves supplies the circular fibres (fig. 530). 



(&) The cervical sympathetic supplies the radiating fibres. The 

 cilio-spinal centre which governs them is in the cervical region of 

 the cord (see p. 721). The fibres leave the cord by the anterior 

 root of the second thoracic nerve, pass into the cervical sympathetic, 



