Hints on spectacles : when required and how to select them / by W. Ackland.

  • Ackland, William.
Date:
[1867]
Catalogue details

Licence: Public Domain Mark

Credit: Hints on spectacles : when required and how to select them / by W. Ackland. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    31/40 (page 31)
    hence 24 inches is the focus of the spectacles that should in this case be used. This plan answers moderately well in those cases where the point of distinct vision is not more distant than 20 or 30 inches, but when long-sightedness has fully developed itself, and the nearest point of distinct vision becomes more distant, the difficulty of estimating the reading distance is so great, that the results obtained are not to be depended on; moreover, by this plan we take no heed of the eye's adjustment, of which more hereafter. Of course,* at the first change from normal to long sight, and in some cases of short sight, it is quite possible to find the most distinct point of vision with all the accuracy we can desire ; but when we have to treat a case of short and weak sight combined, or of long sight of advanced age, it is an impossibility to gain the requi- site information ; moreover, in advanced long sight, this point of vision has so far receded, as to be past the usual range of measurements. In such cases, the employment of a convex lens of suitable focus enables us to so far diminish these, often distant ranges of vision, to measure- able distances; and a knowledge of this fact has given rise to all the various forms of optometers now in use. Unfortunately for the advancement of our knowledge of the focal condition of the eye we examine, the inventors of these most useful instruments have failed to remedy a serious objection to the optometer indications, arising from persons finding it extremely difficult to ascertain the most distinct point of vision, many being enabled to read with equal distinctness over three or four inches of the scale. My plan is not to ascertain by observation the most distinct point of vision on the optometer, but to find