EYE REFRACTION AND ACCOMMODATION 113 



Table 2. Comparison of lens shape and average refractive indices of the 

 ocular media in elasmobranchs and teleosts 



*Charman and Tucker 1973. 



tSivak 1975b. 



^Sivak, unpublished observations. 



light rays must travel further beyond the lens in order to focus on the retina. 

 This greater lens-to-retina distance is apparent in D. sayi (Sivak 1975b). 



Corneal refractive indices (Table 2) are significantly higher than the value 

 reported for teleosts by Charman and Tucker (1973). This is of little refrac- 

 tive importance, since the radii of curvature of the two corneal surfaces 

 are generally very similar. As long as the refractive index of the aqueous 

 humor approaches that of water, most (if not all) of the refractive effect 

 produced by the external corneal surface will be negated by the posterior 

 one. Thus, the interspecies variation of corneal refractive index assumes 

 importance only when the eye is exposed to air. 



DIRECTIONS OF FURTHER RESEARCH 



An understanding of the optical capabilities of the elasmobranch eye will 

 require more persistent study than has been carried out thus far. The 

 results of past and present work, as well as the problems to be overcome, 

 are summarized in the following lines. 



The question of whether elasmobranchs have any accommodative ability 

 is still unanswered. Efforts to induce accommodative changes have pro- 

 duced more failures than successes. It should be recognized that attempts 

 to induce these changes artifically will not solve the problem unless these 

 attempts are accompanied by study of the natural response. For example, 

 in the above study of the dynamic accommodative response of D. sayi, 

 refractive error changes were recorded retinoscopically in response to flash- 

 ing targets presented at varying distances from the eyes of unrestrained 

 specimens (Sivak 1975b). However, the retinoscope is not suitable for 

 recording rapid changes of refractive errors, and other optical methods 



