114 VISION 



should be explored. In teleosts, the visibility of the lens through the cornea 

 permits the use of cinematography (Sivak and Howland 1973). The mobility 

 of the elasmobranch pupil precludes this method, because the lens is not 

 visible. 



Accommodation and refractive error are strictly optical terms that pro- 

 vide little behavioral information. However, both are related directly to 

 resolution ability. Schwassmann and Myer (1971) studied accommodation 

 in teleosts by noting the electrophysiological response of the optic tectum 

 to moving fine wires. They determined visual acuity (by the narrowest 

 wire yielding an optimum response) at various distances from the eye, 

 and from this could infer a refractive error. Accommodative information 

 was yielded by drug-induced refractive error changes. This study is an 

 example of a method for determining the optical quality of the eye from 

 its performance in resolving fine detail. Visual acuity may also be found 

 directly, by the use of conventional behavioral experiments. Both classical 

 and operant conditioning methods have been used in studies of the elasmo- 

 branch visual system. Gruber (1975) examined such psychophysical func- 

 tions as dark adaptation and critical fusion frequency of the lemon shark 

 eye by conditioning the excursion of the nictitating membrane. Graeber 

 and Ebbesson (1972) used a food-reward paradigm to study the ability 

 of nurse sharks to discriminate between horizontal and vertical grid targets. 

 Thus, the combination of behavioral acuity measurements with a conven- 

 tional optical approach appears to be a promising experimental avenue. 



As far as the question of refractive error per se is concerned, the litera- 

 ture review presented here indicates a predominance of hyperopia. It has 

 been suggested that the restricted spectral nature of large bodies of water 

 (blue or blue-green), in conjunction with ocular chromatic aberration, 

 could result in spurious findings of hyperopia (Sivak 1974b). Thus, study 

 of the optical components of the elasmobranch eye should include meas- 

 urement of aberrations. Perhaps the spectral quality of the normal environ- 

 ment should be duplicated during measurement of refractive error. 



Finally, optical study of the elasmobranch eye cannot be separated from 

 other considerations of visual performance. Recent evidence of diurnal 

 visual activity in elasmobranchs (Gruber 1975; Gruber et al. 1975) em- 

 phasizes the need for an adequate description of the eye as an optical 

 instrument. 



ACKNOWLEDGMENTS 



The author wishes to acknowledge the support of the National Research 

 Council of Canada and of the Mote Marine Laboratory, Sarasota, Florida. 



REFERENCES 



Beer, T. 1894. Die Accommodation des Fishauges. Pfluegers Arch. Gesamte 

 Physiol. Menschen Tiere. 58:523-650. 



