128 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



The " broken ring " of Landolt [1876] aims at providing a test less 

 variable than that of the letters. The ring has a diameter which sub- 

 tends 5 minutes of arc at the standard distance, while its width and a 

 gap in the ring each subtend 1 minute of arc. The observer must be 

 able to recognize the position of the gap in any relative position of the 

 letter. Although the measurement of acuity by such a broken ring may 

 be rendered more difficult in the presence of uncorrected visual astig- 

 matism, a greater consistency can be claimed in tests with this type 

 of standardized object. 



Color Variables 



Color is an experienced sensation. The color variables are bright- 

 ness, hue, and saturation. 



At ordinary levels of illumination intensity or of brightness, one can 

 see about the same brightness-difference of colored surfaces as of color- 

 less ones. Between a perfect white and a perfect black one perceives 

 about 60 perceptible shades of neutral grays for an intensity of illumina- 

 tion of about 50 ft-candles. Thus, one can distinguish about 125 hues 

 in the spectrum of sunlight. Hue is a property of color which varies 

 with changes in the frequency of the stimulating light. The ability 

 to detect a difference in saturation (tint) is not very well developed. 

 It is estimated that 20 different degrees of saturation represent the 

 average number of tints of a color actually distinguishable. 



Therefore, multiplying the number of distinguishable hues (125) by 

 the distinguishable shades (30) under a given high intensity of illumina- 

 tion by the distinguishable tints (20) gives 75,000 as the approximate 

 number of different color sensations. If, in addition, the intensity of 

 illumination is changed so as to introduce a range of brightness, it is 

 seen that the number of distinctly different visual sensations which one 

 can experience, excluding those of form, runs into the millions. 



The Fundamental Colors 



It can be shown experimentally that three frequencies may be selected 



o 



from a continuous spectrum — one from the red end at about 6800 A, 



o 



one from the blue end at about 4500 A, and one from the middle at 



o 



about 5560 A — whose combinations in different proportions will give 

 a sensation of white, any of the intermediate shades, or purple. Con- 

 sidered physically, these three frequencies may be designated as funda- 

 mental color arousers, but it is to be remembered that color is a term 

 used to indicate a reaction in consciousness, and it is therefore not 

 strictly applicable to the physical stimuli. 



