Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
N. 286, 287.] 
A. The Adaptation of the Eye 
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 
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.


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