N. 286, 287.]
A. The Adaptation of the Eye
339
however, that for small objects subtending angles of 2° or less the
value of the threshold stimulus of the luminous surface decreases
with the visual angle, but not so fast as to obey Ricco’s law for the
fovea.
In consequence of the fact that the connection between threshold
sensitivity and size of object is distinctly different in light and dark
adaptation, particularly for surfaces of medium size subtending visual
angles between 5° and 15°, the amplitude of adaptation must be
different for luminous areas of different size. In fact, it must increase
with the size of the field. Treitel was aware of this fact. By system¬
atic investigation of the process of adaptation with objects of different
sizes, Piper was able to get considerable differences in the rise of the
curves of adaptation.1
6. Binocular Stimulus Summation2
In determining the threshold of the light sensation, it is not im¬
material whether the observation is made with one eye or with both
eyes. The truth is, in the state of dark adaptation the threshold in
binocular vision is distinctly below what it is in monocular vision;
being about half as high, according to Piper’s measurements.3 This
result has been confirmed in the writer’s laboratory by numerous other
observers. It was corroborated also by W. Lohmann.4 Testing a
number of persons and comparing the results, he found unequivocally
that the increase of sensitivity due to the participation of the other eye
did not occur to the same extent with everybody. The differences were
most marked in the case of subjects who had a squint. This may be
the reason why some observers (Wölfflin5 6, for example) have found
practically the same values of the thresholds of monocular and bin¬
ocular vision. The writer had no trouble in verifying the fact of
binocular summation of stimulus in his own case and in that of numer¬
ous other observers.
1 ^See H. Piéron, De la variation de l’énergie liminaire en fonction de la surface
rétinienne excitée pour la vision périphérique. (Cones et bâtonnets.) Compt. rend. soc. de
biol., LXXXIII. 1921, 753. Also, Des principes physiologiques qui doivent présider à toute
étude de la lumière. Rev. gén. des sei., XXXI. 620 and 656. (H. L.)
2 ffW. de W. Abney and W. Watson, The threshold of vision for different coloured
lights. Phil. Trans. Roy. Soc., 216 A. 1916. 91-142 (see page 109). — P. Reeves, Effect
of size of stimulus and exposure time on retinal threshold. Astrophys. Jour., XLVII. 1918.
141-146. (H. L.)
3 H. Piper, Über das Helligkeitsverhältnis monokular und binokular ausgelöster Licht¬
empfindungen. Zeitschr. f. Psychol, u. Physiol, d. Sinnesorg. XXXII. 161.
4 W. Lohmann, Untersuchungen über Adaptation und ihre Bedeutung für Erkrank¬
ungen des Augenhintergrundes, v. Graefes Arch. f. Ophthal. LXV. 1907.
6 E. Wölfflin, Der Einfluss des Lebensalters auf den Lichtsinn bei dunkeladaptiertem
Auge. Ibid. 61. 1905.