182 On the Structure of the Iris of the Bye of Fishes. 



presents at different times,, possesses a power of dilation, or 

 extension and contraction. In some examples of the fishes on 

 which these observations were made, the cnrtain was only let 

 down sufficiently to hide half the space of the pupil, while in 

 others little of it could be seen until it was brought down into 

 sight by the aid of the point of a needle, and again in others 

 it extended so far as to shut up the whole of the pupil, except 

 what could be discerned between the intervals of the fringe. 



And not only do these curtains present a different appear- 

 ance in different individuals, but even in the same fish this exten- 

 sion of the curtain shall be widely different in each eye, so that in 

 one it shall almost entirely cover the sight, and in the other it may 

 be scarcely capable of being discovered ; a circumstance which 

 goes far to show that the eyes of at least many fishes have a 

 power of vision independent of each other, in a manner or to an 

 extent wholly unlike what we perceive in ourselves, or in any 

 animal of the land. The muscle discovered by Mr. Dalrymple, 

 which influences the position of the crystalline lens of the eye, 

 is present in that organ in the Pleuronectidce , and also in the 

 rays ; but the curtain we have described appears to be the 

 more required in the last-named fishes, from the circumstance 

 that their range of action is often from a considerable depth of 

 water, at the bottom to the broad daylight of the surface. 



In the toper, the pupil is simple, without those appendages 

 which we observe in the rays, and in its form transversely 

 ovoid, furrowed on the surface, and in some examples much 

 larger than in others. A remarkable circumstance connected 

 with the eye of this fish is, that if, when newly taken from its 

 native element, it be laid on its side in such a manner as to 

 keep one of its eyes altogether out of the influence of light, the 

 pupil of the darkened eye will become dilated, while, if the 

 light be strong, the other will become contracted into an irre- 

 gular line, and folds will be discerned in it, radiating more 

 especially to the inferior border, and yet, on its anterior sur- 

 face, there could not be seen any appearance of fibres, nor 

 any special organization beyond the folds produced by the con- 

 traction, which were of a radiating and not a circular character, 

 and those were for the most part near the margin of the pupil. 

 Something similar to this occurs also in the picked dog, but 

 there is so far a difference that the pupil in this species is per- 

 pendicularly ovoid instead of being transverse, and there is no 

 regularity of action in its contraction and dilation as regards 

 the comparison of one with the other, but each of them claims 

 an entire independency, so that uniformity between them is 

 altogether a matter of accident. 



But however difficult it might be found to discover the 

 muscular fibres of the eyes of these fishes while they were 



