. Elementary biophysics: selected topics . Logarithm of average number of photons per flash Fig. 19. This figure presents the experimental findings on three different sub- jects. The lines are the theoretical curves given in the previous figure. As may be appreciated by studying the theoretical curves, the choice of a curve with n greater or smaller by unity would give an appreciably poorer fit to the experi- mental points. (From Hecht, Shlaer, and Pirenne, J. Gen. Physiol. 25, 819, 1942; courtesy the authors and J. Gen. Physiol., Rockefeller Institute Press, Ne
. Elementary biophysics: selected topics . Logarithm of average number of photons per flash Fig. 19. This figure presents the experimental findings on three different sub- jects. The lines are the theoretical curves given in the previous figure. As may be appreciated by studying the theoretical curves, the choice of a curve with n greater or smaller by unity would give an appreciably poorer fit to the experi- mental points. (From Hecht, Shlaer, and Pirenne, J. Gen. Physiol. 25, 819, 1942; courtesy the authors and J. Gen. Physiol., Rockefeller Institute Press, New York.) The theoretical analysis utilizes the Poisson formula. For any average number of incident photons, we can figure out the fraction of times there will be a minimum of, say, 1 photon hitting the retina. This curve is plotted in Fig. 18 with a 1 beside it. Similarly, we can figure out the fraction of times there will be at least 2 photons hitting the retina for any average intensity. And a similar calculation can be carried out for any threshold number of photons. Figure 18 shows these curves for various values of this threshold number. Next w7e try to fit the calculated curves of Fig. 18 to the actual data obtained. Since the shape of the curve depends strongly on the value of the threshold number, it is possible to distinguish quite well between possible curves. In this way, by trying successively 3, 4, 5, 6, 7, etc. photons for the threshold, it is possible to show that the best number is about 6 photons. This means that at low intensities there were few times that the flash was seen because there wrere few flashes which chanced to contain 6 photons, since the average number of photons was less than 6. When an intensity wTas reached such that an average of 6 photons reached the retinal absorbers, not all the flashes were seen because some of the flashes contained 0, 1, 2, 3, 4, and 5 photons, none of which could be seen. When very high intensities were used, all flashe
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