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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    It follows, if we hold before each eye a prism with its base inwards, and look at an object in the middle line, we shall have to render our optic axes more divergent than the true position of the object requires, in order to see it singly; and the strength of the prisms which we can thus overcome by voluntary muscular effort is the measure of our power of voluntary divergence or abduction. In like manner, if we place the prisms with their bases outwards, we shall have to exert our converging power in order to see singly; and the strength of the prisms which we can overcome is in the same way the measure of the converging faculty. Taking as an object a flame at twelve feet distance, I find that I can overcome, for a short time at least, prisms of 22°—that is, two each of 11°, with their bases outwards, and two each of 6^, with their bases inwards. This is with fully relaxed accommodation, wearing convex spectacles of 075 to neutralise my hypermetropia. Great variations of power will be found in different persons, and I lately tested a gentleman who had no abduction or divergence power at all, so that he could not overcome two prisms each of 2° with their bases inwards, but who overcame all I could give him, amounting to an aggregate of 34°, when the bases were turned outwards, and his internal recti muscles came into play. Table III. Overcame by Overcame by adduction abduction (bases outwards). (bases inwards). I . 17° + 17° = 34° 3° + 4° = 7° 2 . 17° + 17° = 34° 5° + 5° =10° 3 • 16° + 16° = 32 5° + 4° = 9° 4 • . 15° + 15° = 30 8° + 8° =16° 5 . 12° + 12° = 24 4° + 4° = S 6 . 10° + 10° = 20° 3° + 4° = 7° 7 • 9° + 9° = 18° 3° + 4° = 7° Table 3 shows the adduction and abduction powers of seven students at St. George's Hospital, taken at random. It will be seen that their adduction power ranged from a minimum of 18° to a maximum of 34°; and their abduction from a minimum of 7° to a maximum of 16°. The object looked at was a flame eight feet