112 VISION 



B 



Figure 2 Vertical asymmetry of the ray eye in Raja batis (Walls 

 1942) and Dasyatis sayi and D. Sabina (Sivak 1975b). 



from the eye. Refractive error was measured with a retinoscope and trial 

 lenses. Refractive errors became more myopic (or less hyperopic) as target 

 distance decreased, the amount of change corresponding to the difference 

 in the vergence of incident light rays. Since these changes were found along 

 a specific axis of the eye, a dynamic accommodative ability is indicated. 

 In view of the existence of dynamic accommodation, the dorsal lengthen- 

 ing of the visual axis of the eye as an accommodative mechanism must be 

 questioned. 



Refractive Components of the Eye 



Except for work on the dorsal asymmetry of the ray eye (Franz 1934, 

 Sivak 1975b) and comments by Rochon-Duvigneaud (1943) on the asphe- 

 ricity of the elasmobranch lens, the refractive components of the elasmo- 

 branch eye have been ignored. Authors such as Wells (1942) and Verrier 

 (1930) have simply extrapolated from the teleosts. For example, they 

 characterize both groups as having spherical lenses of high refractive index. 

 This description, stemming largely from an early study of the optical nature 

 of the teleost eye (Matthiessen 1880), is frequently considered to apply to 

 all aquatic vertebrates. 



Preliminary measurements of intraocular dimensions and refractive indices 

 of the ocular media (Sivak 19756; Sivak, unpublished observations) indicate 

 that the optical components of the elasmobranch eye are quite different 

 than those of the teleost (Table 2). Intraocular measurements were made 

 following a rapid freezing and sectioning procedure. All measurements of 

 refractive index were made with an Abbe refractometer. As noted earlier 

 by Rochon-Duvigneaud (1943), the elasmobranch lens is not spherical. 

 For example, the average vertical lens diameter of Dasyatis sayi is 18% 

 longer than the horizontal diameter. Refractive indices of elasmobranch 

 lenses appear to be significantly lower than the equivalent value of 1.69 

 reported for the goldfish (Charman and Tucker 1973). Lens indices, in 

 elasmobranchs, were found by squeezing the lenses between the refractom- 

 eter prisms, a procedure which is not possible with the teleost lens. The 

 asphericity of the lens and its lower index of refraction mean that incident 



