VIII

CONCLUSION

In summarizing the results of this study, attention is directed to
the following points:

1.   As regards the relation of acuity to intensity, it is shown that
with uncolored illumination approximately 75 per cent, of daylight
acuity is attained with an intensity of from 8 to 10 meter-candles;
with a reduction of intensity below this point the acuity decreases
rapidly, and with an increase of intensity beyond this point the
acuity rises very slowly, unit acuity being attained with an inten¬
sity of from 40 to 50 meter-candles.

As in daylight vision, after unit acuity is attained, further in¬
crease of intensity shows practically no gain in acuity. It may
therefore be considered that intensities of 8 and 40 meter-candles
constitute approximately the lower and upper limits, respectively, of
suitable illumination for ordinary purposes.

2.   As to the relative efficiency of red and green or blue illumina¬
tion, the advantage lies decidedly with the red. Monochromatic yel¬
low does not enter into this comparison. So-called white illumina¬
tion gives a slightly higher acuity than red, but inasmuch as white
illumination is predominantly yellow, it is quite possible that mono¬
chromatic yellow would have the same influence on acuity as white.

3.   A probable explanation of the greater efficiency of red illumi¬
nation is to be found in the apparently greater sensitivity of the form
perceiving end-organs, the cones, to light of longer wave-length, as
compared with the brightness perceiving elements, the rods, which
have been shown to be more sensitive to the shorter wave-lengths.

4.   In the study of visual acuity many distinct factors are in¬
volved, failure to regard any of which will seriously affect the results
obtained.   Among these the most important are:

(a) Photometric determinations.—Accurate estimation of rela¬
tive intensities of colored illumination by direct methods is exceed¬
ingly difficult, if not impossible, and the flicker method is proposed
as the only satisfactory one for making heterochromatic comparisons;

(5) Test-characters.—Care should be taken that the test-char¬
acter employed actually measures the form sense rather than the
brightness sense, and that it involves the use of the eye under condi-

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