II

REVIEW OF PREVIOUS WORK IN VISUAL ACUITY

According to Nagel,^ the first careful study of the relation of
acuity to intensity of illumination was made by Tobias Mayer, in the
year 1754. His method of observation is not given, but as a result
of his investigations he formulated the law that acuity varies as the
sixth root of the intensity of illumination.

In 1871, more than a century later, Cohn carried on a series of
tests with untrained subjects in ordinary daylight, varying the
intensity of the illumination by means of the Weber Polarisation-
episkotister. Comparing his own results with those of Mayer, Posch,
Albertotti, Sous and Carp, Cohn asserted as his conclusion that
^ ^ enormous individual differences in visual acuity are found with the
decrease in the intensity of illumination, and we are yet far from
the formulation of a law for their correlation."^

Cohn makes the remarkable statement that he found some eyes
that had unit acuity with an illumination of only 1.5 meter-candles,
and half acuity with only .6 of a meter-candle. Of all eyes tested,
full acuity was attained on the average at 16 meter-candles and half
acuity at 4 meter-candles.

Cohn found that it was practically useless to make observations
with daylight as the source of illumination, inasmuch as the eye
does not by any means show the differences which the photometer
shows. That is, within certain limits great variations in the inten¬
sity of the illumination are not attended with any noticeable differ¬
ences in the acuity. Thus for an acuity of 1 the intensity varied to
as much as ten times the minimum value; for an acuity of .75 the
intensity varied to 12 times the minimum value; and for an acuity
of .5 it varied to 7 times the minimum.

Dissatisfied with the enormous variations which he found, Cohn
repeated his observations with a great number of persons and took
the average of all the readings thus obtained. Assuming unit
acuity for 100 units of intensity, an acuity of .75 was obtained with
71 units of intensity, and an acuity of .50 with 33 units of intensity.

In 1876 Posch^ asserted as the conclusion of a series of observa-

*'' Handbuch der Physiol, des Menschen,'' III., 342.
^Archiv fiir Ophthalmologic, 1871, 17, (2), 305.
* Archiv filr Augenheillcunde, 1876, 5, (1), 14.

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