The Effect of Red Fatigue on the White Equation. 



233 



of the portion of spectrum in the green for the normal equation is given 



above, and corresponds to thirteen scale divisions. The equation is very 



-easy to make, the mean deviation being very small. 



The following are the results of twelve consecutive observations by two 



normal-sighted persons, the equations being made alternately, starting with 



itoo much or too little green in the equation : — 



A. From green... 13 From red... 13 E. From green... 13 From red... 12*5 

 13 13 12-5 13 



13 13-5 12-5 13 



One scale division at \ 515 corresponds to about 1 A. 



The screens, which are viewed at a distance of 4 feet, so that the images 

 shall fall on the foveae, are coated with magnesium oxide. The experiments 

 were made in order to find out the effect of fatigue with red light, which is 

 supposed to affect only the hypothetical red sensation. 



The light used for fatigue was a " Pointolite " arc of 100 c.p. with a con- 

 denser, viewed through a deep red glass, which only allows red from \ 630 ^ijx 

 to the end of the red end of the spectrum to pass. The light was viewed with 

 both eyes, and care was taken that the arc should not be directly visible. 

 An ordinary incandescent electric lamp obscured by ground glass was also 

 used. Red isolated in my spectrometer of the region \ 670 was also used 

 with the same result. After a certain amount of fatigue, the length of time 

 varying with different persons, the mixed and the simple lights were viewed. 

 There was now a striking change in the mixed white, which appeared a 

 brilliant green, and no longer matched the simple white. In order to match 

 the simple white, the green had to be so reduced that the mixed light 

 appeared red to a person with an unfatigued eye. Several normal-sighted 

 persons were then tried with the same result. Observations were taken with 

 the equation correct for the observer, and with too much or too little green 

 in the equation. My normal equation is : 36 red, 14 green, and 42 violet ; 

 after 5 seconds' red fatigue, the equation was : red 36, green 7, violet 42. 



The following observations are by two normal-sighted persons. A, normal 

 equation : red 36, green 13, violet 42 ; after 30 seconds' red fatigue, the 

 equation was : red 36, green 8, violet 42, and the equation made was exactly 

 the same, whether the normal equation was viewed or an equation in which 

 there was too much or too little green. The observer did not know which 

 was shown. B, normal equation: red 36, green 12, violet 42; after 

 20 seconds' red fatigue, the equation was : red 36, green 5, violet 42, when 

 his normal equation was viewed ; red 36, green 5^, violet 42, when the 

 ■equation viewed at first contained too much green ; and red 36, green 4, 

 violet 42, when the equation viewed at first contained too little green. 



