. The clinical use of prisms; and the decentering of lenses . position ofthe two eyes. When ordinary spectacles are pre-scribed for eyes placed unsymmetrically, theprismatic effect is greater for one than the other,which, however, is only of importance with verystrong lenses, being counteracted by involuntaryinclination of the head; another consequence isthat the fields of fixation for each eye areunsymmetrical. i56 PRISMS. TRIAL-FRAME FOR TEMPORARY the first edition of this work it was said thatprisms should not be prescribed without wearinga temporary pair for a few evenings first,
. The clinical use of prisms; and the decentering of lenses . position ofthe two eyes. When ordinary spectacles are pre-scribed for eyes placed unsymmetrically, theprismatic effect is greater for one than the other,which, however, is only of importance with verystrong lenses, being counteracted by involuntaryinclination of the head; another consequence isthat the fields of fixation for each eye areunsymmetrical. i56 PRISMS. TRIAL-FRAME FOR TEMPORARY the first edition of this work it was said thatprisms should not be prescribed without wearinga temporary pair for a few evenings first, to learnhow they suit, but I now think this rarely needful,for if the conditions of convergence be carefullystudied we need hardly ever prescribe prisms withoutfeeling pretty sure beforehand of their effects, eventhough, as is often equally the case with cylinders,they do not seem well borne just at first. Still,there are occasions where temporary prisms are ofservice, and the diagram of the frame to hold themmay be reproduced, Fig. 66. It is made to hold. Fig. 66.—Rectangular prisms in a frame as suggested in the frame may be obtained from Messrs. Curry & Paxton. rectangular* prisms, the shape of which permitsthem to be adjusted at once, and removes anypossibility of their displacement. Each prism hasa longitudinal groove in its upper and lower sur-face, so that it need only be pushed in between thewires till caught by the elbow. So light a frame asthis could be worn together with an ordinary pair * Dr. Percival has proposed a slight modification, namely,that the prisms should be square instead of oblong. LOCALISING THE EYES. 157 of spectacles when it is desired to try for a fewevenings the joint effect of prismatic and sphericalglasses.* I would recommend, in prescribing prisms,the following simple maxims : 1.—Never order strong prisms unless the indica-tions for their use are unmistakable. 2.—Rarely order them stronger than 40 (2°d)unless there be amblyopia
Size: 3236px × 772px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisher, booksubjectlenses
