312 Fly-rods and Fly-tackle. 



the nerves which convey the impression to the brain, or 

 of the brain itself, but by a simple mechanical adjustment 

 of the lens which forms the image. If the rays of light 

 proceed from a distant object, they strike the lens when 

 substantially parallel, and it has nothing to do but to 

 converge them to a focus. If, however, they proceed 

 from an object close at hand, they then strike the lens 

 while diverging, and must first be made parallel, and 

 afterwards converged to a focus, before a distinct image 

 can be formed. Obviously, then, the focal point in the 

 second case will be farther from the lens than in that 

 first given. In the telescope this is adjusted by varying 

 the distance between the object-glass and the eye-piece, 

 while in the human eye an involuntary alteration of the 

 convexity of its lens accomplishes that result. Unless 

 this adjustment is possible, the human eye cannot and 

 does not give distinct vision at all distances. It is not 

 possible in all individuals, and then near-sightedness or 

 far-sightedness follows the aid of a compensating lens 

 is required to perform this adjustment, and spectacles 

 must be employed. If the anatomy of the human eye 

 teaches this, and it is beyond question that it does, an 

 examination of the structure of the trout's eye should 

 give at least some indication of its powers. 



The lens of the human eye has the ordinary lens form, 

 and is a little more convex on the inner than on the 

 outer side ; and by a contraction or expansion of its di- 

 ameter, thus changing its convexity and consequently its 

 refracting power, does it adapt itself when in health al- 

 ways to form its focus or in other words, to produce a 

 distinct image at the same point, the retina. 



Does the eye of the trout possess this or any equiva- 

 lent property ? It does not. The lens is as spherical as 



