The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . a typical case of asthenopia. Ophthalmometer. — Astigmatism with the rule, 2 D., axis 45°+ or 135° — right eye ; astigmatism against the rule, 2D.,axis 135° + or 45° - left eye. Perhaps it is well here again to refer to the fact that whentin- two chief meridians of curvature fall exactly at 45° and 135°,just halfway between the vertical and horizontal meridians,that, strictly speaking, wre do not have astigmatism with oragainst the rule. But for the sake of uniformit
The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . a typical case of asthenopia. Ophthalmometer. — Astigmatism with the rule, 2 D., axis 45°+ or 135° — right eye ; astigmatism against the rule, 2D.,axis 135° + or 45° - left eye. Perhaps it is well here again to refer to the fact that whentin- two chief meridians of curvature fall exactly at 45° and 135°,just halfway between the vertical and horizontal meridians,that, strictly speaking, wre do not have astigmatism with oragainst the rule. But for the sake of uniformity of reading of 76 THE REFRACTION OF THE EYE the instrument we speak of these cases as being with andagainst the rule, just as in other cases. Furthermore, to make the reading of the instrument uniform,we have assumed one or the other of the meridians, either 45°or 135°, always as the primary position or starting point. Formyself I always take 135° as the primary position in such with 135° as the primary position and turning thelong indicator to 45°, that is at right angles, if the mires over-. R. E. L. E. Fig. 40. lap we call the astigmatism with the rule, just as at other timeswhen the mires overlap; and if the mires separate when wereach 45° we call it astigmatism against the rule, approximatethe mires a second time, and turn back to the primary positionat 135° to get the amount of overlapping. It must be remembered, too, in these cases with the axes at45° and 135°, that the subtraction of .50 D. from the readingof the instrument when the astigmatism is with the rule andthe addition of .50 D. to the reading when it is against therule does not always hold. ILLUSTRATIVE CASES 77 In the present case in the right eye the lines dividing themires became straight with each other at L35 ( primary position);the images of the mires were then approximated and turned to4> (secondary position), when two steps of overlapping tookplace; astigmatism with
Size: 2054px × 1217px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthordavisa, bookcentury1900, bookdecade1900, booksubjecteye
