CHAPTER XXVII 



EXPERIMENTS ON THE DIOPTRIC MECHANISM 



AN eye (ox, sheep, or pig) is to be dissected. (1) After cleaning away from the 

 globe all remains of muscles, fat, etc., cut a window out from the back, removing 

 the sclera and choroid and exposing the retina. Notice that when the cornea 

 is turned to the window an inverted image of this is formed upon the retina. 

 (2) Cut away a small portion of the sclera at the edge of the cornea. A grey- 

 looking ring of plain muscular tissue is exposed, the fibres passing from the 

 corneo-sclerotic junction backwards over and into the choroid. This is the 

 ciliary muscle. (3) Cut the eye in two at its equator. Notice, in the posterior 

 half, from which the jelly-like vitreous humour flows away, the retina usually 

 somewhat opaque and crumpled after death spreading out from the entrance 

 of the optic nerve ; in the anterior half the lens within its capsule, the suspensory 

 ligament around the margin of the lens, the radiating ciliary processes. (4) Snip 

 through the suspensory ligament all round the lens, which can be removed 

 within its capsule ; the iris is now seen projecting into the anterior chamber. 



Accommodation; Change in shape of the lens. That the lens 

 bulges forward in accommodation is shown in various ways. 



1. Stand at the side of another person and let him fix his vision 

 on a distant object, looking beyond a near object such as a needle or 

 pencil held a few inches from the eye. Notice his iris which can be 

 seen through the edge of the cornea lying against the front of the lens. 

 Now let the subject look at the near object. His iris is seen to advance, 

 being pushed forwards by the bulging lens ; the pupil at the same 

 time contracts. 



2. Sansoris images. In a dark room hold a candle at one side of 

 the eye of a subject, and, standing on the other side, observe the 

 reflected images a bright one from the front of the cornea, a less 

 bright one from the front of the lens, and a duller, small, and inverted 

 image difficult to see from the back of the lens. The subject as 

 before is to have his vision fixed at first on a distant object, and is 

 then to transfer his gaze to a near object in the same line. The 

 image reflected from the front of the lens becomes smaller and moves 

 nearer to that reflected from the front of the cornea ; the other 



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