SENSE ORGANS. m 



This is not definitely settled, but we are again in- 

 debted to Helmholtz for the most probable view, viz., 

 that it is done by contraction of the ciliary muscle. We 

 have already mentioned (page 103) the lens capsule, its 

 continuation as a curtain outward all around, and its 

 attachment to the sclerotic a little behind the iris. Now 

 this curtain is taut, and therefore the capsule presses 

 gently on the elastic lens and flattens it. This is the 

 passive condition of the eye when it is accommodated 

 to distant objects. Now there is a muscular collar about 

 the iris, on the inside of the sclerotic, the fibers of which, 

 arising from the outer margin of the iris, radiate out- 

 ward and backward, and, taking hold of the outer margin 

 of the lens curtain where it is attached to the sclerotic, 

 pulls it forward to where the circumference is less, and 

 therefore slackens its tautness and allows the elastic lens 

 to bulge. The amount of bulging is in proportion to 

 the slackening, which will be in proportion to the con- 

 traction, and this in proportion to the nearness of the 

 object. 



See, then : the eye is more like the microscope, in 

 that it changes the lens rather than removes the screen. 

 But how much more perfect! The microscope has its 

 four-inch lens, its two-inch lens, its one-inch, its half- 

 inch, its tenth-inch lens, and changes one for another 

 as the object is nearer. The eye has but one lens, 

 but it changes the form of its one lens so as to make it 

 a six-inch lens, a foot lens, a twenty-foot lens, a mile 

 lens, or a million-mile lens, for at all these distances it 

 makes a perfect image. 



4. Adjustment for Light.— In both the camera and 

 the eye some contrivance is wanted to regulate the 

 amount of light admitted. In both, too, this is done by 

 diaphragms with holes of varying size. In the eye the 

 iris is the diaphragm and the pupil the hole. But in this 

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