Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale.

Adolf Gaston Eugen Fick

Date:: 1896

Credit: Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale. Source: Wellcome Collection.

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$these holes; the observer, sitting at a proper distance before the cards and trying to read, will at once notice that when reading the near type the farther will look as if covered with tissue paper and is unreadable, while if the eye looks through a hole at a word on the far card, the near type is indistinct. This change in the eye, called Accommodation, can be brought about in two ways, either by displacement of the retina backward (in Figs. I and 2 toward the right), or by a change in the dioptric apparatus, this last necessarily implying a change of refractive strength, or, in other words, a shortening of the focal distance. That accommodation for near objects by displacement of the retina backward (lengthening of the bulb) can take place has been asserted by various ophthalmologists recently. Schneller among the number. Considering the softness of most eyes and the circum- stance that a displacement of the retina of only 0.6 mm. backward would suffice to accommodate an emmetropic eye from infinity to 0.5 m., this idea is not altogether improbable. Nevertheless, even the advocate of this idea confesses that a backward displacement of the retina can take place-only with the severest strain on accommo- dation and convergence, which is not a normal process. It can be said, therefore, with all certainty, that this extension of the axis during ordinary accommodation plays no part. If accommodation for a near object is brought about in the diop- tric apparatus, the following possibilities must be considered:— (1) Increase of the refractive index. (2) Advance of the lens, that is, its approach toward the cornea. (3) Increased curvature of the refracting surfaces. The first possibility need not be examined, since the short time necessary for accommodation gives no chance for a change in the density of the refractive media. The second possibility, accommo- dation through advance of the lens toward the cornea, cannot be so lightly dismissed, and has had, as a matter of fact, numerous advocates of the highest scientific rank. But exact experiments show that although the anterior lens surface does approach the cornea during accommodation, this slight change of position is never enough to account for the reduced focal distance that takes place. A positive visible displacement of the lens will be neces- sary, and as this does not happen we can dismiss the idea. The third possibility remains, accommodation by means of in- creased curvature of the refractive surfaces of the cornea and lens.$