Variability of Accommodation during Steady Fixation at Various Levels of Illuminance*

Using the stigmatoscope, the variability of the dioptric power of the eye during steady fixation was measured for four observers over a range of intensities from $\overline{2}.5$ to 3.0 log trolands in one-half logarithmic steps. Two different viewing conditions, (constant and variable size test letters), were studied but no significant difference between them was obtained. There was almost a fourfold decrease in the variability of the settings as the intensity was increased over this range. There was a sharp transition at one troland, suggestive of a discontinuity between scotopic and photopic vision. These changes could be eliminated by cycloplegia of the fixating eye. Essentially the same results were obtained when the refraction of one eye was measured objectively (with a concidence optometer), while the other eye fixated the chart. This latter technique is, however, less valid at the lower intensities because of the tendency to fixate the measuring light of the optometer. The data can be quantitatively described by a theory which postulates that the accommodation is continually fluctuating and that the limits of fluctuation are proportional to threshold ΔI/I.