HO VISION 



eyes (or bending the head or body or both) to produce a new and longer 

 visual axis. It is important to emphasize that light rays emanating from 

 such a target will require a longer focal distance (Fig. 1). In effect, this 

 mechanism will produce the same refractive result as one in which the 

 lens moves away from the retina. According to Franz, the asymmetry 

 of the ray eye (Raja batis) is such that a transfer of image position from 

 the central to the dorsal retina will fulfill an accommodative function. 



PRESENT RESEARCH 



Accommodation 



The accommodative mechanism involving the protactor lentis has gained 

 widespread acceptance (Prince 1956; Duke-Elder 1958; Tansley 1965). 

 This is because of its acceptance by Walls (1942) and because it tidily re- 

 lates refractive error (hyperopia) to accommodation. An additional factor 

 is the lack, until very recently, of any subsequent experimentation. 



Somiya and Tamura (1973) include some elasmobranchs in a photo- 

 graphic study of lens motion induced electrically in excised teleost eyes. 

 While a lack of movement was reported, it should be noted that the 

 mobility of the elasmobranch pupil may make the position of the lens 

 difficult to record photographically. A recent attempt to induce refrac- 

 tive changes in the eye of the lemon shark by the use of drugs failed as 



B 



Figure 1 Focal conditions of an eye, indicat- 

 ing that (A) when distant targets are focused 

 on the retina the eye is hyperopic at nearer 

 points, and (B) when near targets are focused 

 on the retina the eye is myopic for distant 

 points. 



