On defects of vision which are remediable by Optical appliances : A course of lectures delivered at the Royal College of Surgeons of England.

  • Carter, Robert Brudenell, 1828-1918.
Date:
1877
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Licence: Public Domain Mark

Credit: On defects of vision which are remediable by Optical appliances : A course of lectures delivered at the Royal College of Surgeons of England. 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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    wholly changed. The object, instead of lying at the intersection of the optic axes, and at the line of division of the positive from the negative accommodation, was beyond the intersection, and wholly within the negative part of the range, as shown in Diagram B, Fig. 19. These changes in the conditions of the relative accommodation are exactly similar in kind, and very nearly iden- tical in degree, with those which would follow if the eyes were rendered convergent to twelve inches naturally, instead of by means of prisms. When stronger prisms were employed—say of 10°—the changes, which took place were the samiC in kind, but greater in degree. A convex of 1*25 dioptrics was then the strongest glass through which the object remained distinct, but a concave of 3*50 could be overcome, making the total relative accommodation again equal to 4-25, This is shown in C, Fig. 19. When the prisms were 12*5 the convex glass was 075, and the concave 4*0, the entire relative accommodation being 475, as shown in D, Fig. 19, When the prisms were 15, the accommodation could not be relaxed at all, even for so weak a glass as 0*50, which made vision indistinct. Hence the positive part of the relative ac- commodation became constantly smaller as the strength of the prisms was increased; until (in E, Fig. 19) the accommodation is all negative. And whereas, in the first diagram, the object was in the middle of the range, or at the very beginning of the negative portion of the accommodation, it ultimately came to lie at the farthest extremity of this portion. If now, instead of increasing his convergence by turning the bases of the prisms outward. Dr. Loring lessened it by turning them inward, a result identical in kind followed, only the optic axes intersected farther from the eyes, the relative accommodation was likewise removed, and the proportions between its positive and negative portions modified. Fig. 20 represents the various changes produced by the prisms, the bases being placed inwards. Weaker prisms were employed in this case, as the optic axes are rendered parallel by prisms the united angles of which amount to 15°. Consequently, those were used whose united angles amounted to 5°, 10°, and 15°; instead, as in the former case, of 10°, 20*^, and 30^. It will