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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    it obviates the harassing and unsatisfactory process of trying back- wards and forwards with concave and convex cylindrical glasses under circumstances which make it almost certain that the patient’s answers will become confused ; and also that it dispenses entirely with the necessity of resorting to the use of mydriatics for the purposes of subjective optometry. APPENDIX. It may be appropriate to add here a somewhat fuller explanation of certain phenomena which characterise astigmic vision in the outer zones. It is hardly necessary to observe that the diagrams printed with the text are by no means intended to indicate that the vertical or horizontal elements (as the case may be) disappear the moment the object is carried out of the mean zone. On the contrary, a distinct factor in the trouble and uncertainty of astigmic vision is the persist- ance, at certain ranges, of blurred or doubled elements lying in one meridian in combination with fairly distinct elements in the transverse meridian. It would be approximately correct to say, if we take as the unit of visibility an object subtending an angle of five minutes at the retina, that each unit may be expected to continue to be more or less visible for a range of 2 or 3D. beyond the punctum rewotum, and prob- ably for a relative distance within the punctum, proximum, although in the latter case it is, under ordinary circumstances, difficult to determine the vanishing point with accuracy because of the rapid increase in the apparent size of any given object.* The diagrammatic representation of the zones of vision in an astigmic eye, such as that to which Diagram I. refers, might, therefore, be more justly represented by some such scheme as the following: — Distance in metres from the eye .. .. 0.16 0.20 0.25 0.33 0.5 1 Dioptres .... 6 5 4 3 2 1 0 —1 —2 Remote Mean zone zone zone Diagram IV. * The determination may of course be made if desired by using a series of test objects decreasing proportionately in magnitude as each approaches the eye. I have constructed such an optometer for my own use, and have found it of some service.