The velocity of accommodation / by James W. Barrington.

  • James Barrett
Date:
[1885]
Catalogue details

Licence: Public Domain Mark

Credit: The velocity of accommodation / by James W. Barrington. Source: Wellcome Collection.

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

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    values that I w ould especially direct attention, since the errors for nearly all observations are alike. In order to measure the time taken to relax the accommodation the reverse experiment is performed. The ocular is first depressed and the observer accommodates for the image which is distant 35 cm.; he then suddeidy elevates the ocular into the line of vision, and, as soon as the image becomes clear, again depresses it. The recorded interval between the raising and depression of the lever represents the duration of the rehixation period together with the time taken for the nervous changes. In all cases where one eye was used alone, a large blinker was placed over the other eye, which was allowed to remain open; for the ordinary practice of shutting the eyelids of one eye and keeping the other open was found to alter the result very materially, since there is a great tendency to accompany the closing of the lids by contraction of the ciliary muscle. It was found that if the eyelids be closed and then opened, the accommodation is nearly always completely relaxed. In fact, when the writer was estimating his own accommodation velocity, he at times found difficulty (especially if fatigued) in relaxing fully in any other way. In all cases a definite period was allowed to elapse during which the object Avas looked at prior to the observation being com- menced ; this varied from 5 to 10 sees. The following is the method by which the measurements have been made. Determination of the rate of accommodation from far to near. To measure the time required to change from infinity to 33*3cm. an ocular (+ 4D) whose focal distance is 25 cm. is fixed at 25 cm. from the image formed by the objective. The eye of the observer is fixed at 8 cm. from the ocular. Then the ocular is depressed and the image thrown out of focus. The time at whicli the image again becomes clear, is then recorded by the lever being raised. In this case the ciliary muscle contracts through -i- 3D, i.e. adds on to the crystalline lens + 3D. To measure from near to far, the person accommodates for the image, while the ocular is depressed: he then raises the lens into the axis of vision, and, as soon as the image is clear, again depresses it to record the time when the act is completed. To measure accommodation velocity from infinity to 11 cm. The ocular is a lens (+ 20D) the focal distance of which is 5 cm. and the eye is at 6 cm. from ocular. The experiments were conducted as before. By modifications of oculars and of the distance of the head from the
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