||J. Chariton and C. A. Lea Some Experiments concerning the Counting of Scintillations Produced by Alpha Particles. Part I Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character Vol. 122, No. 789 (Jan. 1, 1929), pp. 304-319 link AND von Kries, J., and Eyster, J. A. E., Uber die zur Erregung des Sehorgans erforderlichen Energiemenzen, Z. Sinnesphysiol., 1907, 41, 394. AND Barnes, R. B., and Czerny, M., Lasst sich ein Schroteffekt der Photonen mit dem Auge beobachten?, Z. Physik., 1932, 79, 436.
||p.820 bottom paragraph:"From these twelve researches, [investigators] have chosen the three sets of measurements which are free from what can now be recognized as obvious error. These are given in Table I. Even though they differ by a factor of about 3, these data can be considered as roughly confirming one another. However, since for [their] purposes a factor of 3 cannot be ignored, [they] undertook to make the measurements again, but under the best physical and physiological conditions."