Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice.

  • Prentice, Charles F. (Charles Frederick), 1854-
Date:
1907
Catalogue details

Licence: In copyright

Credit: Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice. 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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    total accommodation required at 25 cm. is used in reading at 33J cm. distance, because the corrections then more closely correspond to Donders’ table, and also for the reason that the author has found it more satisfactory to thus calculate in practice. For instance, the emmetrope, whose ampli¬ tude of accommodation is 2.5 D., should have 3 D. of refraction to read at 33^ cm., but, as he must also have one-third of the accommodation required at this distance in reserve, he should have 4 D. of refraction to read comfortably. Since his dynamic refraction is 2.5 D. we therefore give him + 1.5 D. glasses to supply the deficiency. If his range of accommodation is normal, we find, by the chart, that he should be 50 years of age. In this manner the author frequently estimates the ages of persons with surprising accuracy. The lenticular corrections, inserted in the field of the chart, between the ages of 45 and 80 years, added to the corresponding amplitude of accom¬ modation, less the acquired hyperopia noted beneath, are found in each instance to amount to 4 D. Consequently, in any case, the requisite 3 D. to read at 33£ cm. represents three-fourths of the total power supplied both dynamically and artificially, by lenses. Of course, the corrections in the table, are only applicable in such cases where the reading distance is no greater than 33^ cm., and wherein tin- amplitude of accommodation is found to correspond to Donders’ deter¬ minations. The chief value of the chart therefore exists only in the fact that it serves to show, by comparison with any given case under examination, to what extent its amplitude of accommodation differs from the accepted normal state as determined by Donders. This cannot be too highly estimated, however, for Landolt * says: ' ‘ Donders’ diagram corresponds so perfectly to the natural condition of things, that, in every case where the amplitude of accommodation is less than is indicated thereon, we may safely diagnose a weakness of accommodation, and, in case of any considerable difference, we may admit a paresis of this function.” A knowledge of the patient s condition of health and habits will of course assist greatly in arriving at a definite decision. The punclum proximnm is that point located at finite distance, at which * Landolt’s work, page 55G.