712 



VERHOEFF AND BELL. 



are those of the three normal color sensations reduced to equal areas 

 as determined by Exner. If any of these primary sensations are 

 fatigued that of the remaining color or colors becomes the predomi- 

 nant tint seen. This has been beautifully shown by Burch who by 

 suitable means fatigued to complete exhaustion each one of the sen- 

 sations and various combinations of them. Burch ^^ found that of 

 the three the red first regained its sensibility, after perhaps ten min- 

 utes, followed by the green and last of all by the blue where marked 

 fatigue might persist for several hours. Red vision is therefore 

 normally to l)e expected after fatigue of all three sensations since the 

 red recovers first and in case the green and blue are more fatigued than 

 the red the latter will be more notably predominant. As the maxi- 

 mum luminosity of the spectrum lies in the green and as at high alti- 

 tudes under a clear sky the blue is relatively strong, exposure in the 



Wave length 

 Figure 6. Primarv color sensations, after Exner. 



high snow fields necessarily fatigues these two sensations predomi- 

 nantly, and photoerythropsia in greater or less degree may reasonably 

 be expected. 



As a corollary we may note the reputed activity of the quartz mer- 

 cury arc in producing erythropsia. On figure 2, Plate 5, marked in 

 their proper positions are the three chief lines of the mercury spectrum 

 at waves lengths 454 /x/x, 546 /xjUj and the pair at 578 ix]x. It will be 

 seen that they lie in positions which indicate stronger fatigue of the 

 green than of the red and marked fatigue in the blue. The green line 

 at 546 \x\x is by far the strongest of the three followed by the yellow 

 pair at 578 /x^u and by the strong line of blue. The chief red line in the 

 spectrum is relatively ver\' weak hence fatigue weakens the green most, 

 red in the next degree and blue relatively little. After fatigue, 



