The Influence of Intensity, Color and Retinal Location on the Fusion Frequency of Intermittent Illumination.

1. Nature of Problem.-The flickering sensation produced by regularly interrupted illumination disappears when the frequency of interruptions is made sufficiently high. Under controlled conditions the determination of this critical fusion frequency may be made with considerable accuracy; its precise value, however, depends on a variety of conditions, of which the most effective is the intensity of the illumination. The relation between these two variables is not wholly clear at present. Though the dependence of the critical frequency on illumination was recognized over one hundred years ago, it was only forty years ago that Ferry' formulated what has since become known as the Ferry-Porter law, namely, that the critical frequency is proportional to the logarithm of the illumination intensity. Ferry's published measurements distinctly do not support this generalization, but the later data of Porter2 do. Porter, however, found that when the critical fusion frequency-as cycles of light and dark per second-is plotted against the logarithm of the intensity the data fall on two straight lines instead of one. The two lines intersect at an illumination of about 0.25 meter candles, and the slope of the lower is 1.56 while that of the upper is 12.4. These findings were corroborated by Ives3 in 1912. Ives's data for different parts of the spectrum show a dual logarithmic relation similar to that for white light. However, the slope of the straight lines and their point of intersection seem to vary with the wave-length of the light, the upper and lower limbs of the relationship varying in different ways. In addition Ives found the extraordinary fact that for blue light, the lower line becomes horizontal. These peculiarities are difficult to reconcile with the obvious interpretation of Porter's data which is generally given in terms of the Duplicity Theory, that is, that the lower limb describes the function of the rods while the upper limb describes the function of the cones. This difficulty has been emphasized by Allen4 whose flicker studies in general confirm Porter and Ives, though differing from them in particulars. Thus Allen draws through his measurements about five short straight lines of different slope, instead of the two drawn by Porter and by Ives. In our estimation, the data presented by Allen do not justify such treatment. The points seem to lie on a continuously curving line, and deviate from it no more than would be expected when special precautions are