The accomodation of the eye to distances / by William Clay Wallace.

  • Wallace, William Clay
Date:
1850
Catalogue details

Licence: Public Domain Mark

Credit: The accomodation of the eye to distances / by William Clay Wallace. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    8/40 page 6
    The hypothesis of Crampton, that the form of the cornea could be altered by the contractions of a muscle at its circumference, cannot be maintained, for the fibres which he discovered in the eyes of birds are so arranged that the effect could not be produced. 2. Alteration of the Diameter of the Pupil.—As the eye is constructed on the same principle as the camera ob- scura, it is evident that alteration of the diameter of the pupil can have no more effect in producing in the eye a distinct image of objects at different distances, than alteration of the diameter of the aperture for the admis- sion of light can produce a well-defined picture in a camera obscura. The diameter of the pupil is changed with every variation of light, yet the focus is not dis- turbed ; and on looking through a pin-hole in a card, adaptation to distance is just as necessary as in vision with the naked eye. 3. Muscularity of the Crystalline Lens.—The fibres of the crystalline, prismatic in form and brittle in consist- ence, are totally different from muscular structure; and if they really did possess contractility, there is no point of attachment from which the fibres could act. In a certain species of hawk the crystalline is a plano-convex, and in all animals it is so exquisitely cut, if I may use the ex- pression, that the irregular contraction of muscles would produce irregular refraction. In aquatic animals, in which the crystalline is very dense, the fibres are separated with so much difficulty that it is not probable they could slide over each other in such a manner as to produce the supposed effect. Sir D. Brewster has ascertained that in these animals the fibres of the crystalline are dovetailed into ^'fftheLTil'.Tf ^^^^ other by a species of teeth, which would the Cod. prevgnt any change in the configuration of the From Rojr'!!. refracting raedium.
    digitised image 1
    digitised image 2
    digitised image 3