256 



TABLES 290-291. 

 THE EYE AND RADIATION- 



Definitions: A meter-candle is the intensity of illumination due to a standard candle at a meter distance. The 

 millilambert (o.ooi lambert) measures the brightness of a perfectly diffusing (according to Lamberts cosine law) 

 surface diffusing i lumen per cm j . A brightness of 10 meter-candles equals i millilambert. o.ooi ml corresponds 

 roughly to night exteriors, o.i, to night interiors, 10 ml to daylight interiors and 1000, to daylight exteriors. A bright- 

 ness of 100,000 meter-candles Is about that of a horizontal plane for summer day with sun in zenith, 500, on a cloudy 

 day, 4, ist magnitude stars just visible, 0.2, full moon in zenith, .001, by starlight; in winter the intensity at noon may 

 drop about $. 



TABLE 290. Spectral Variation of Sensitiveness as a Function of Intensity. 



Radiation is easily visible to most eyes from 0.330 ft (violet) to 0.770 M (red). At low intensities near threshold 

 values (gray, rod vision) the maximum of spectral sensibility lies near 0.503 /J. (green) for 90% of all persons. At higher 

 intensities after the establishment of cone vision, the max. shifts as far as 0.560 u. See Table 297 for more accurate 

 values of sensitiveness after this shift has been accomplished. The ratio of optical sensation to the intensity of energy 

 increases with increasing energy more rapidly for the red than for the shorter wave-lengths (Purkmje phenomenon); 

 i.e., a red light of equal intensity to the eye with a green one will appear darker as the intensities are equally lowered. 

 This phenomenon disappears above a certain intensity (above 10 millilamberts). Table due to Nutting, Bulletin 

 Bureau of Standards. 



The intensity is given for the spectrum at 0.535/1 (green). 



TABLE 291. Threshold Sensibility as Related to Field Brightness. 



The eye perceives with ease and comfort a billion-fold range of intensities. The following data were obtained with 

 the eye fully adapted to the sensitizing field, B, the field flashed off, and immediately the intensity, T, of a test spot 

 (angular size at eye about 5) adjusted to be just visible. This table gives a measure of the brightness, T, necessary 

 to just pick up objects when the eye is adapted to a brightness, B. Intensities are indicated log intensities in milli- 

 lamberts. Blanchard, Physical Review, n, p. 81, 1918. 



SMITHSONIAN TABLES. 



