On the vision of objects on and in the eye / by William Mackenzie.

  • William Mackenzie
Date:
1845
Catalogue details

Licence: Public Domain Mark

Credit: On the vision of objects on and in the eye / by William Mackenzie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    19/64 (page 19)
    § £]. Real motions of the pearly muscae. volitantes. If a musca volitans is in or near the axis of vision, it is easy for the patient to keep it fixed for a length of time, over any particu- lar spot in the prospect before him, or in the centre of the lumi- nous field presented by a candle, viewed through a convex or con- cave lens. The pearly muscae volitantes, however, have a motion of their own, excited no doubt by the motions of the eye-ball, but still more extensive than their apparent motions, and partly exhibited after the eye-ball is at rest. Thus, if, from looking be- fore him in a horizontal direction, the patient suddenly raises his eyes towards a point about 30° above the horizon, and fixes them on some object at that height, he observes that the muscae fly up- wards considerably beyond that degree of elevation, and even be- yond the field of view, and then come sailing down before him till they disappear below, evidently showing that whatever be the nature and seat of the corpuscles by which such spectra are pro- duced, they are not fixed, but enjoy a certain degree of freedom. Muscae volitantes are described by some authors* as suddenly darting upwards, and then as suddenly sinking ; but they never do this, unless first set in motion by the movement of the eye- ball; and although the motion which they perform in consequence of the impetus given them by the movement of the eye-ball is rapid, that by which they return to their former place is,compa- ratively slow. Were the corpuscles, which produce the pearly muscae volitan- ! tes, situated anterior to the focal centre of the eye, then their real motion would correspond with the apparent motion of the spectrum or muscae; that is to say, they would descend within the eye, when the muscae appeared to descend; but if the corpus- cles be situated behind the focal centre, the apparent descent of the muscae must depend on an actual ascent of the corpuscles. Wherever a corpuscle is situated, whether at a or at a', (Fig. 4,) its spectrum will appear as if projected out of the eye in the con- tinued course of a straight line, passing through the corpuscle, and falling nearly perpendicularly on the retina at A. A corpuscle at b, anterior to the focal centre of the eye, or at b\ posterior to that centre, will form its image at B, and produce the sensation of a spectrum out of the eye, in the direction of the line Bb'b. If the corpuscle is situated at a, and sinks in the eye to b, then its image will move over the retina from A to B, and its spectrum will appear to the patient to descend from A' to B\ But if the corpuscle is situated at o', it will only be by floating upwards ifrom o' to b' in the vitreous humour that its image will°move over j e retina from A to B, and its spectrum appear to the patient to i descend from A' to B'. Beer, Lehre von den Augenkrankheiten, vol. ii. p. 424. Wien, IB]7.