44 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



thicknesses of black cloth to prevent leakage of light. The cardboard 

 front of the fixation-lantern was pierced by a circular aperture 5 mm. 

 in diameter. This point of light served as the fixation-object. Its 

 yellowness was neutralized by the insertion of appropriate blue gela- 

 tines, and its brightness was reduced by the addition of white tissue 

 papers. The luminosity of the fixation-point was slight, but it was well 

 above the limen of perceptibility. 



A circular aperture 15 mm. in diameter was cut in the cardboard 

 front of the stimulus-lantern. The color stimuli were produced by in- 

 serting immediately behind the plane of this aperture appropriate com- 

 binations of translucent sheets of colored gelatine. The light-stimulus 

 transmitted through the gelatines could be controlled at will, in bright- 

 ness, in saturation, and in color tone, by an appropriate choice of the 

 gelatines which constituted the combination. 



It was intended, at the outset, to reproduce exactly the experi- 

 mental conditions employed by Hellpach. This plan, unfortunately, 

 proved to be impracticable, for the reason that Hellpach's descriptions 

 are so vague and inaccurate as to render an exact duplication of his 

 experiments impossible. For example, instead of making a direct state- 

 ment of the wave-lengths of his color stimuli, he gives the positions of 

 the Frauenhofer lines in his spectrum, and the scale-readings which he 

 obtained in the spectroscopic examination of his stimuli. It should, of 

 course, be a simple matter to chart his spectrum from the former data, 

 and by a method of graphic interpolation to obtain a regular curve 

 which would enable us to correlate the scale-readings of his stimuli 

 with the wave-lengths which enter into their composition. But Hell- 

 pach's data refuse to submit to being charted. Instead of giving a 

 regular curve which sweeps down with approximately constant change 

 of direction from the red to the violet, they give an impossible figure, 

 shaped somewhat like an elongated W, with its central crest lying in 

 the region of the E and b lines. It is impossible to determine with any 

 degree of definiteness the height of any intermediate point upon this 

 irregular curve. One is unable, therefore, to discover, from his data, 

 what were the wave-lengths of the stimuli employed in his experiments.* 



Since we were balked in our attempt to duplicate Hellpach's colors, 



*The values which Hellpach reports for the positions of his Frauenhofer 

 lines are clearly erroneous. And since the spectral determinations of his stimuli 

 refer to these erroneously determined constants, the data contained in his paper 

 furnish no definite information as to the wave-lengths of his color-stimuli. It is 

 true that in one case that of his yellow stimulus he records his spectral deter- 

 mination in unequivocal terms. But the testimony of Peters, and our own ex- 

 perience with this stimulus, show that here, too, Hellpach has made a false deter- 

 mination. 



