284 nature's teachings. 



already seen how the pressure of water upon the front of the 

 eye lessens its convexity, and makes it long-sighted. Conse- 

 quently, if we could apply pressure round it, we could make it 

 more convex, and so neutralise the weight of the water. 



There is a wonderful piece of machinery in Nature which 

 really does perform this office, the eye, at the will of its owner, 

 becoming either telescopic or microscopic. This quality is very 

 desirable in birds, especially those which are predacious and of 

 rapid flight, as they might either fail to see their prey at a 

 distance, or might dash themselves against some obstacle when 

 they were close upon it. 



The eye of the Owl affords a beautiful example of machinery 

 which produces this effect, and the means which are used may 

 be understood by inspecting the accompanying illustration. 



It will be seen that the eyeball is set in a framework composed 

 of thin bony plates, just like a glass in a telescope. When these 

 plates are relaxed, the whole eyeball is flattened, so as to enable 

 the bird to see an object at a very great distance. But, when they 

 are contracted, they render the whole eye globular in propor- 

 tion to their pressure, and enable the bird to see objects which 

 are very close to it. In fact, the eye becomes a telescope or 

 microscope as needed. 



Many reptiles possess this arrangement of bones, but the 

 birds have even a more delicate mode of obtaining the focus of 

 the eye. This is by means of a curious organ called, from its 

 shape, the " pecten," or comb, which is placed in the vitreous 

 humour at the back of the eye, and connected with the optic 

 nerve. It is a congeries of arteries and veins, so that it can be 

 rapidly enlarged by forcing blood into it, or diminished by 

 allowing the blood to withdraw. 



As the liquid in which it rests is practically incompressible, 

 it follows that when the comb expands, it causes the chamber 

 of the vitreous fluid to expand, and so forces the lens forward. 

 When, however, the blood retires from the comb, the lens 

 returns to its original place. This, as the reader may have 

 noticed, is the same principle as that which is followed in alter- 

 ing the focus of a telescope in order to suit the sight of different 

 individuals. Perhaps a still better illustration may be found in 

 the coarse and fine adjustment of the microscope, the former of 

 which moves the whole tube, and may be compared to the bony 



