Artificial Colour-hlindness to Successive Contrast. 211 



across the entire spectrum, but only that part of it corresponding to 

 the colour-sensation excited by the monochromatic light. The effect 

 is very striking after red light. An intensely black band cuts through 

 the spectrum from the ultra-red, as far as C, where it begins to fade 

 away into a pure green. 



After violet light, a similar black band cuts through the spectrum 

 from the ultra-violet, and if care has been taken not to implicate the 

 blue in the fatigue, the black band fades away into blue. 



After green light, most frequently the red and blue are seen to 

 stretch across and meet in the middle of the b lines ; but sometimes, 

 if the exposure is exactly right, a well-marked darkening of that part 

 of the spectrum is seen. 



Blue light is the most difficult to manage, unless a wide dispersion is 

 used, the blue being otherwise not sufficiently separated from the green 

 and the violet. After getting the adjustments right, it is better either 

 to wait ten minutes, or use the other eye. With these precautions, it is 

 easy to see the green and violet meet in the place of the blue, and to 

 note that the after-image of the blue casts no shadow on the violet 

 near H. Sometimes a momentary shadow may be seen in the blue. 

 This experiment is of interest as affording additional evidence of the 

 existence of a separate sensation for blue. 



It should be noted that the two spectra must have the correspond- 

 ing colours on the same side. If the prism of the second spectro- 

 scope is reversed so as to bring the red of one spectrum towards the 

 violet of the other, a black shade, very well defined, can be produced 

 in any part of the second spectrum. For if the two spectra are so 

 arranged that any given portion of the one corresponds with the same 

 wave-length on the other, then no part of the one spectrum on either 

 side of that one part will be of equal wave-length with the portion of 

 the other spectrum which coincides with it. Accordingly, a negative 

 after-image will be produced only of the short space within which the 

 wave-lengths are approximately the same. But it is clear that such 

 an experiment is more curious than useful. 



III. Contrast Phenomena by Intermittent Stimulation. 



In 1868 Sigmund Exner* made a series of experiments on the fol- 

 lowing plan. After an interval of darkness, he presented to the eye, 

 for a fraction of a second, the image of a small white disc. This was 

 succeeded by a disc of considerably larger diameter, which in turn was 

 followed by darkness. The illumination of either disc, and the period 

 during which it was visible, could be independently varied. Thus the 

 portion of the retina on which fell the image of the small disc received 



* Exner,* Sitzungsbcriclite d. Wiener Akad.' Abth. 2, vol. 58 (1868), pp. 601 

 —632. 



