108 VISION 



INTRODUCTION 



Three areas of concern have dominated the comparative study of visual 

 optics: (a) measurement of refractive error, or the degree to which the 

 focal point of light entering the eye fails to coincide with the retina, 

 (b) accommodation, or the ability of the eye to vary its focusing power 

 to maintain image quality for varying fixation distances, and (c) the study of 

 the individual optical components of the eye, including intraocular distances, 

 radii of curvature, and refractive indices. 



The optical quality of the elasmobranch eye has been the subject of only 

 five published articles. Moreover, this minute body of literature is remark- 

 ably contradictory. 



LITERATURE 



Beer 



Beer (1894) included a small section on elasmobranchs in his voluminous 

 study of the teleost eye. He noted that the mobility of the elasmobranch 

 iris makes measurement of refractive error difficult. In specimens in which 

 a measurement was possible, several diopters of hyperopia were reported. 

 Beer's findings were made with the direct ophthalmoscope, an instrument 

 he assumed was limited by reflection from the vitreous surface of the retina. 

 All final values for teleosts include a compensatory subtraction based on 

 the thickness of the retina. This procedure produced characteristic findings 

 of myopia and led to a controversy concerning the refractive nature of the 

 teleost eye (Verrier 1928; Sivak 1974b, 1975a). Beer did not report whether 

 the same considerations were involved in his determination of hyperopia 

 in elasmobranchs. 



Beer also attempted to induce accommodative refractive changes by 

 electrically stimulating enucleated elasmobranch eyes. In the teleost eye, 

 such simulation caused the lens to move toward the retina. No change was 

 noted in elasmobranchs. Beer did not rule out the possibility of an accom- 

 modative ability in this vertebrate class. However, he relegated vision to 

 the position of a secondary sense in elasmobranchs and considered olfac- 

 tion dominant. 



Franz vs Verrier 



Aside from a brief report by Franz (1905) concerning a failure to electri- 

 cally induce lens movement, the first studies directed specifically at the 

 optics of the elasmobranch eye were those of Verrier (1930) and Franz 

 (1931). Both commented at length' on the difficulties encountered in re- 

 fracting elasmobranchs. These include (a) the mobility of the pupil and 

 (b) the interposition of large pectoral fins between the eye and an aquarium 

 wall in the case of skates and rays. The retinoscope was used in both studies, 

 although Franz also verified his findings with an ophthalmoscope. In each 



