358 



COLOR AND LIGHTING 



in which the violet rays come to a focus far- 

 ther to the front of the eye than the red rays, 

 and the yellow raj^s occupy an intermediate 

 position. These intermediate rays are auto- 

 matically focussed upon the retina. This 

 results in aberration circles for the red and 

 violet which produce a slight blurring of the 

 optical image, and consequently a reduction 

 in visual acuity. This blurring is not notice- 

 able in ordinary circumstances because the 

 brightness of the yellow rays is much greater 

 than that of the violet or the red. Further- 

 more, the eye is much more sensitive to 

 yellow than to red or violet. Consequently 

 the aberration circles are seldom noticed. 

 We might expect, however, that vision would 

 be clearer if all rays but those from one re- 

 gion in the spectrum were eliminated. That 

 is, the more nearly monochromatic a light, 

 the sharper the image on the retina. 



Comparison of Various Illuminants 



Visual acuity varies measurably from one 

 monochromatic light to another. Ferree 

 and Rand's careful determinations (10) 

 found the order yielding greatest to least 

 acuity to be: yellow, yellow-green, orange, 

 green, red, blue-green and blue. But sun- 

 light was superior to any of the colors. 



Sodium versus Tungsten Light 



Luckiesh and Moss (16) measured visual 

 acuity under sodium vapor and under tung- 

 sten filament light. For all practical pur- 

 poses, the sodium light was monochromatic. 

 The brightness range was from 0.018 to 20 

 millilamberts. Acuity of nine subjects was 

 measured by means of the Ives-Cobb acuity 

 object. At all levels of brightness, the so- 

 dium light produced greater acuity. Speed 

 of visual reaction was found, in the same 

 study, to be greater under sodium than under 

 tungsten light when the test object is small, 

 the brightness one millilambert, and the con- 

 trast between object and ground large. But 

 when the brightness was only 0.1 millilam- 

 bert, the test object large, and the brightness 

 contrast very small, there was no difference 



in speed. Incidentally, it has been demon- 

 strated by Luckiesh and Holladay (14, p. 

 246) that sodium j^ellow light does not pene- 

 trate fog appreciably better than white light. 

 It is apparent from these studies that where 

 very fine (threshold) discrimination is con- 

 cerned, sodium vapor light is slightly better 

 than ordinary tungsten. However, as dem- 

 onstrated by Luckiesh (14, p. 246), this ad- 

 vantage for sodium light or any monochro- 

 matic light disappears when objects to be 

 discriminated are well above threshold size. 

 For example, visibility of book print (8 to 10 

 point) is about the same under sodium and 

 under tungsten light. Sodium light is un- 

 desirable for general use because all color but 

 yellow disappears. It has no advantage in 

 enhancing contrasts (14, p. 246). 



Mercury Arc Light versus Tungsten Filament 

 Light 



It was commonly held that in view of the 

 sharp retinal image obtained with monochro- 

 matic light (as referred to above), visual 

 acuity would be improved the nearer one gets 

 to monochromatic illumination. (It might 

 be pointed out that some people consider 

 that there is no rational argument for this 

 hypothesis.) Therefore, mercury arc light, 

 which has relatively homogeneous wave 

 lengths, should produce greater visual acuity 

 than tungsten filament light. Luckiesh and 

 Moss (18, p. 406) question the validity of 

 early experiments on this subject. They 

 (17) measured visual acuity, under 5, 25, and 

 125 foot-candles of illumination from the low- 

 pressure mercury arc and a gas filled tung- 

 sten-filament lamp. Measurements were 

 obtained from 10 subjects on the Ives-Cobb 

 acuity object. At each foot-candle level 

 there were only insignificant differences be- 

 tween the lights in visual acuity. They 

 conclude that the mercury light produces no 

 greater visual acuity because of less chro- 

 matic aberration than the tungsten light 

 which has a continuous spectrum. 



In another experiment, Luckiesh and Moss 

 (18, p. 418) report that the visibility of book 



