The systematic use of cylinders in making the shadow test / by Alexander Duane.
- Alexander Duane
- Date:
- 1903
Licence: In copyright
Credit: The systematic use of cylinders in making the shadow test / by Alexander Duane. 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.
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![band of light), and then again moving mirror in the meridian of 165°, see if reversal of the shadow has yet been secured. If not I replace tlie +3.00 with stronger cylinders. Finally with a +3.50 cylinder I find that I just succeed in making the shadow go with the mirror in the meridian of 165°. If this cylinder of +3.50 at 75° truly represents the amount and axis of astigmatism, then I should, with my two reversing glasses (+2.75 spherical 3 +3.50 cylinder), get the following: (a) At one metre there should be a bright circular reflex uniformly illuminating the whole pupil. (h) The shadow should move with tlie mirror in all meridians alike and precisely in the same line as the mirror moves, not swerv- ing from the path of tlie latter; that is, not making any oblique movement. (c) Wlien I advance to just within one metre the shadow should begin to move against the mirror in all meridians and for all alike at precisely the same distance from the eye. If my correction of the astigmatism is ivroiig in amount, e. g., if the cylinder should l)e +3.25 instead of +3.50, tlien, as I ap- •proacli the ])atient from a distance of one metre, I shall at a certain distance from him find that in the meridian of 165° the shadow moves with the mirror, wliile in tlie meridian of 75° it still moves against it. When this happens I simply change the strength of the cylin- der until reversal takes place at just the same distance from the eye for all meridians alike. If my cylinder’ is at the wrong axis, i. e., if it ought to be at 80° instead of 75°, then as I sweep tlie mirror from side to side or up and down I will notice that the shadow, instead of travel- ing along the same line as that in which I am moving my mirror, makes a skew or oblique movement, sliding off, as it were, to one side or the other. When this happens I shift the axis of my cylinder one way or the other, until this obliquity of movement disappears. Finally, if my spherdcal alone is at fault then reversal takes place evenly indeed in all meridians, but either too close to the eye or too far from it. I then alter the strength of the spherical accordingly, until reversal takes place at just one metre. In making the test in this way it is essential for the observer to get into the habit of constantly varying, his distance from the patient—moving his head slightly backward and forward so as to be sometimes just within a metre^s distance from the patient, some- times just beyond it. I regard this forward-and-back movement of](https://iiif.wellcomecollection.org/image/b22449668_0006.jp2/full/800%2C/0/default.jpg)
