On defects of vision which are remediable by Optical appliances : a course of lectures delivered at the Royal College of Surgeons of England.

  • Carter, Robert Brudenell, 1828-1918.
Date:
1877
Catalogue details

Licence: Public Domain Mark

Credit: On defects of vision which are remediable by Optical appliances : a course of lectures delivered at the Royal College of Surgeons of England. 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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    small objects, in order that they may be clearly seen, must be held so far away that they either subtend a visual angle which is too small to allow them to be recognised with facility, or else sufficient light for the requirements of vision is no longer reflected from their surfaces. The amount of light reflected from any point, and falling upon a given area, such as the pupil of the eye, varies, it need hardly be said, as the square of the distance between the two ; so that an object which is only twelve inches distant gives the pupil four times as much light as it would do if it were removed to a position twenty-four inches away. Hence, those who are becom- ing presbyopic in an inconvenient degree first feel the effect of the change by twilight, or by artificial light of any kind, which, however good it may be, and however dazzling when near the eyes, never approaches in its general illuminating effect to that of even a feeble degree of diffuse daylight. By the age of forty-live, or thereabouts, it is generally necessary for the possesser of em- metropic eyes to hold very small print so far away (especially in the evening) that it becomes difficult to decipher. It will help us to realise the improvements which have been made, during the present century, in the means of artificial lighting, if I remind you of Dr. Kitchener's well-known saying, that people who were fifty years old began to bless the man who invented snuffers. At the same period, or somewhat later, we often see an artificial light held between the reader and his page, so as to increase the illumination of the latter to the utmost, without unduly taxing the accommodation. The term presbyopia, in former times, has been used with a good deal of latitude, or, rather, with a complete absence of any- thing like scientific precision ; but now, in accordance with the teaching of Bonders, it is used only to signify the alteration of vision which is produced by the recession of the near-point, or the gradual curtailment of the power of accommodation, which is incidental to advancing life. It is evident, therefore, that the term, used in this sense, cannot be defined, except in an arbitrary manner; for the degree of presbyopia which is felt as an incon- venience will be mainly determined by the nature of individual