656 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the sclera around the cornea with a thin fluid layer 

 between the contact lens and the cornea. 



Specification of Amount of AccommodatKin m Play 



Accommodation is measured in diopters and repre- 

 sents the reciprocal of the distance from the conjugate 

 focus of the retina to the spectacle point when the 

 distance correction is worn. Thus when the distance 

 correction is worn the accommodation must be re- 

 duced to its zero level in order to see a distant object 

 clearly. 



The ma.ximal amount of accommodation that can 

 be elicited is called the amplitude of accommodation. 

 The nearest point for which the eye can accommodate 

 is called the near point of accommodation (^punctum 

 proxirnurtt). This varies with age as shown in figure 15. 

 At the age of about 40, the near point recedes rapidly 

 beyond the ordinary working distance of 33 to 40 cm. 

 \'arious sets of data relating amplitude to age have 

 been analyzed by Marg et al. (63). It is necessary to 

 compensate for this loss of accommodation with a 

 plus lens added to the distance correction. The added 

 plus lens power at the near point is usuallv pro\ided 

 in the form of a bifocal which has enough plus lens 

 power added in the segment to permit the eye to see 

 clearly at a given working distance with one half of 

 the accommodation held in reserve. 



Ap/iafcia 



When the crystalline lens is missing, the eye is 

 said to be aphakic. The lens may be surgically re- 

 moved, it may be congenitally absent, or it may be 

 dislocated so that it fails to cover the pupillary aper- 

 ture. Clear vision can still be obtained by means of a 

 lens mounted in front of it, but the eye no longer has 

 any power of accommodation. The range of clear 

 vision through any given lens is strictly a function of 

 the depth of the focus of the eye. 



FIG. 15. Regression of the 

 near point of accommodation 

 with age. Based on Donder's 

 data (20). It is assumed that the 

 distance correction is worn. 



20 30 40 



iGE (YEARS) 



OCUL.AR ME.ASUREMENTS 



In the first section of this chapter reference was 

 made to the indices of the media and the curvature 

 of the refracting surfaces and their distances in front 

 ot the retina. In the second section, reference was 

 made to the distance from the conjugate focus of the 

 retina to the spectacle point and its use in specifying 

 refraction and accommodation. This section explains 

 how such quantities are actually measured. 



Indices of Media 



Each medium of the eye has been assumed to be 

 uniform in the schematic eye; but the cornea and the 

 lens have a definite microstructure and can be 

 treated as made up of layers of different indices. 



The cornea has five different layers, the epithelium, 

 Bowman's membrane, the stroma, Descemet's mem- 

 brane and the endothelium. The index for the 

 epithelium has been found to be 1.416 and that for 

 the rest of the cornea, 1.372 (22, p. 728). 



The index of the lens substance varies from 1.387 

 at the cortex to 1.406 at the center (22, p. 736). The 

 variation in index of the lens from the center to the 

 cortex affects its performance as an image-forming 

 device. The problem can be formulated by visualizing 

 a series of isoindical surfaces from the cortex to the 

 center. In a meridian section these surfaces probably 

 correspond to the course of the fibers as they arch 

 around the nucleus from the axis back to the axis 



(79. P- 339)- 



The index of the aqueous is 1.336 and that of the 



vitreous is the same (22, p. 734). 



The index measurements reported above ha\e all 



been made with an Abbe refractometer which makes 



use of the principle of total reflection. 



Purkinje Images 



The Purkinje images are important because they 

 provide us with many objective methods of studying 

 the configuration, tilt and location of the refracting 

 surfaces. The refracting surfaces of the eye can form 

 images both by reflection and refraction. Images in- 

 volving single reflection at the front and iiack surfaces 

 of the cornea and at the front and iiack surfaces of the 

 lens are known as the first, second, third and fourth 

 Purkinje images (86, p. 48). These images may be 

 seen bv an observer located in front of the eye. 



