The effect of various wave lengths of visible light in the stimulation of single visual sense cells has been studied by means of the single fiber preparation from the eye of Limulus. Oscillographic records were made of the impulse discharge in a single optic nerve fiber in response to stimulation of the attached sense cell by lights of different wave lengths. Wratten monochromatic filters supplied the means for obtaining the different spectral lights; the total intensity supplied to the eye being determined by a thermopile and galvanometer.
With lights of approximately equal energy content the strongest response occurs to the green region of the spectrum. The response, however, does not vary qualitatively with wave length. By the proper adjustment of intensity, responses can be obtained which are identical, impulse for impulse, for all the spectral lights used. Moreover the ratios of the intensities for the various wave lengths necessary to produce a constant response do not vary with the intensity level of the stimulating lights; there is no Purkinje effect. The single visual sense cell can gauge brightness but cannot distinguish wave length.
The reciprocals of the intensities necessary to produce a constant response when plotted against wave length give the visibility curve for the single sense cell. This curve is symmetrical about a maximum at λ520mµ, falling off to low values in the red and violet. It closely resembles the visibility curve for human rod vision.
Bundles from the optic nerve containing several active fibers whose impulses can be distinguished by differences in form and magnitude or whose attached sense cells can be located and illuminated independently were used to determine whether there is any differential sensitivity among sense cells in the same eye for different regions of the spectrum. Such a differential sensitivity has been found to exist in the eye of Limulus and may be considered a peripheral mechanism of color vision.