Optometry by the subjective method : a paper read at the meeting of the British Medical Association, London, July, 1895 / by George J. Bull.

  • Bull, George Joseph, 1848-
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Optometry by the subjective method : a paper read at the meeting of the British Medical Association, London, July, 1895 / by George J. Bull. 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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    describe their vision of any of the larger letters in the test-types by saying that they see it as if there were several letters superposed in such a manner that the main image in the centre would be overlaid on the one side by one or more faint reduplications, and on another side by a similar series. Such a person (supposing the astigmia to be direct) may for example in looking at the large C, observe a faint letter displaced upwards and another faint letter displaced down- wards, and if he does he will probably notice that the area where these images coincide with the position of the actual letter in the centre, .becomes prominent as a very black and striking portion of the figure seen. One cause of the difficulty which an astigmic patient experiences in seeing such a letter correctly is that the shape of this prominent portion of the field may be entirely different from the shape of the original letter. It would, however, be misleading to suggest that these appearances are specially characteristic of astigmia, as dis- tinguished from other errors of refraction, since the apparent partial superposition of multiple images may be observed in other forms of refrac tive error. It is of considerable importance for practical as well as theoretic purposes that these phenomena should be accurately observed, and this is not always easy. I have found, for example, that astigmic patients often find it difficult to answer definitely as to the relative values of the lines of the clock, apparently because of the somewhat puzzling changes through which the different meridians pass rapidly as the patient observes them through a consecutive series of lenses. This observation has led me for my own convenience to substitute for the clock-face a simpler, but in my experience a more delicate and reliable instrument. This consists of a rotatory disc, carrying only two lines at right angles to each other, which I call the cross. I find that the patient, having only two lines to compare, is able to observe slight doublings or shadows, or slight differences in the intensity and definition of these two lines, much more accurately than he can observe the precise differences of value between the various lines of the clock. The way in which I am accustomed to work is to first determine roughly with the clock, using spherical lenses only, the general direction of the meridian of least refraction, and then to ascertain more closely by means of the cross the exact position in which this meridian lies. At this point I direct the patient’s attention to the line of the cross perceived by his meridian of least refraction, and correct that meridian carefully with a spherical lens. When this is accomplished, it is comparatively easy to find the concave cylindrical glass by which the transverse line of the cross is made equally definite and intense.