On the Adjustment of the Eye, &'c. 341 



every time we view objects at different distances. Nor does 

 it appear probable that the power of adjustment lies either 

 in the vitreous humour or in the crystalUne lens, for they 

 are too dense to admit of that infinite variety of changes 

 which the eye undergoes every day. 



As the cryslallinc lens alone is sufficient to form a distinct 

 image of an object upon the retina, it is therefore verv 

 reasonable to suppose that the aqueous and the vitreous 

 humours arc designed for other purposes than merely to 

 act as refracting mediums. 



The vitreous humour is a dense transparent substance, 

 which seems designed by nature to preserve the lenticular 

 form of the crystalline, and to keep it at the same distance 

 from the retina ; for the crystalline lens is so deeply imbed- 

 ded in the vitreous humour, and adheres so closely to it, 

 that the crystalline cannot suffer any change in its form, 

 without giving motion to some part of the vitreous. 



Now, as nature performs all her mechanical operations 

 by the most perfect mechanical laws, it must appear im- 

 probable, from the density of these substances, that any 

 change should take place, either in the figure of the crystal- 

 line lens or in its distance from the retina. 



Just before the crystalline lens the iris is placed. Tlie use 

 of this organ is to contract and enlarge the pupil ; and as it 

 is put in motion by almost every ray of light that falls upon 

 the cornea, it seems necessary for its preservation that it 

 should meet with the least resistance possible ; therefore it 

 swims in a limpid fluid called the aqueous humour. 



The contraction of the pupil answers two important pur- 

 poses ; one to exclude too great a quantity of light, which 

 would be injurious to the eye, and the other to give us a 

 distinct view of near objects. But how a contracted pupil 

 should produce this effect may be explained from t}ie fol- 

 lowing property of vision, viz. — 



That we have no vision perfectly distinct, but by rays 

 which enter the eye in a direction parallel to the axis of the 

 crystalline lens. 



This will be easily understood from the following obser- 

 vations : 



Y 3 First 



