148 DE. COBBOLD ON THE EYE OF THE COD-FISH. 



attention to the first only of these complicated layers, I have to re- 

 mark that in the Cod, as also in its allies, we find Jacob's membrane 

 to consist of rods and cones, the latter presenting the characteristic 

 twin or double form well known to occur in many fishes. In my 

 opinion this twin character is a normal condition ; but this view is 

 opposed by Mr. JSTunneley, who has arrived at very different con- 

 clusions not only in this particular, but also in regard to their size 

 and other easily ascertainable facts. According to my own ex- 

 aminations, the twin-cones of the Cod present an average length 

 of 5-Jo* n of an inch in length, and -g^th of an inch in breadth ; 

 but I have found some only g^o^h °^ an l° n g> an( i others as 

 much as ^^th after they have imbibed fluids added to them. 

 In the fresh condition they display the form shown in the accom- 

 panying woodcut («), having, as Hannover has remarked, the shape 



o b c f i 



Twin*COHe8 of flic retina of Hie Cod, showing flic formal ion of bacillar appen- 

 dages, and the changes of form which the cones undergo on the addition of 

 water. 



of a coffee-berry, the upper or inner pole of the twin-cone being 

 somewhal more broadly truncated than the lower or outer pole, 



$ 



