The action and uses of prismatic combinations / by Archibald Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
[1881]
Catalogue details

Licence: Public Domain Mark

Credit: The action and uses of prismatic combinations / by Archibald Percival. 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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    IO obtained over a relatively small area of the whole fixation field, for the relations of the double images become entirely different in its different parts ; and it is impracticable to adapt the prisms to these changes, as has been pointed out by Dr. Noyes. It is then chiefly, in the second class of cases, in those in which the ränge of movement is represented by an angle of iS° or more that prismatic combinations may be ordered to be constantly worn, and here they are only applicable when the error is not greater than 2° in each eye. lf, for instance, the maximum divergence be 4°, instead of the normal 2°, and the maximum conver- gence be 15°, abducting prisms of 2° d may be pre- scribed, which will have the effect of placing the ränge in a more available Situation ; in fact, its limits will now be — 2° and + 17°. Should the patient be a hyper- metrope of + 5 D, prismatic combinations of + 5 D convex withi°44'^ will be sufficient, as the deviating action of this combination is the same as that of a plane prism {vide Table I.) of 2° d. If the defect exceed the stated limit of 2°, tenotomy of the preponderating muscles is indicated.* I have here assumed that muscular errors, due to displacement of the ränge, should be corrected by alter- ing its position to such an extent that the far point of convergence may occupy its normal site. It remains to be seen whether this assumption is justifiable. The question now opened up is a difficult one, and demands detailed investigation. Even after the most careful examination of the con- dition of the ocular muscles, comprising the measurement of the relative ränge, both when the aecommodation is relaxed and when it is exerted for the workingdistance, * Tenotomy has the disadvantage of diminishing the amplitude 01 movement, so thatunless the ränge is of the normal extent, advancement of the feeble muscles would be preferable, as by that means the ränge is increased in amplitude, while it is also rendered more available by the alteration of its position.