116 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



TABLE III-4 

 Pupillary Diameter Changes with Changes in Brightness 

 Brightness units B in millilamberts 

 Pupillary diameters D in millimeters (average of 6 subjects) 



By Courtesy of P. Reeves [1918], and the Eastman Kodak Company. 



were photographed and the dimensions were increased 7 per cent so as 

 to obtain the actual dimensions. Under these circumstances the pupil- 

 lary diameters shown in Table III— 4 were obtained. However, we can 

 draw only an approximate conclusion from these results, namely, that 

 the logarithm of the brightness times the pupillary area is nearly con- 

 stant for brightness greater than 10.0 millilamberts. Reeves' work also 

 shows that the response of the pupil under conditions of artificial light- 

 ing is such that at 2000 millilamberts the diameter of the pupil is as 

 small as 2 mm. This contraction of the pupil is comparable to that 

 produced when a 100-watt lamp in a 5-in. diameter globe of good diffus- 

 ing glass is held near the eye. 



Spectral Sensitivity of the Eye 



The quantitative study of the response of the retina to light of various 

 wavelengths originated with R. A. Konig as early as 1893. His data 

 ef visibility at low intensities are used in Fig. Ill— 9. The curve of visi- 

 bility at high intensities, as adopted by the Illuminating Engineering 

 Society to represent the composite results of Ives, Kingsbury, Nutting, 

 Coblentz, Emerson, Hyde, Cody, and Forsythe, are also shown for 

 comparison. 



The visibility at low intensities was determined by a threshold method, 

 which consists in determining the least amount of energy that is just 

 perceptible at each wavelength. The visibility at high intensities is 

 determined by means of a direct measurement of the relative brightness 

 of equal amounts of energy throughout the spectrum. From these 

 curves it will be observed that the sensitivity of the eye shifts towards 

 the blue end of the spectrum at low levels of illumination. This shift 

 is attributed to a transition from cone to rod vision. Hence, if a red 

 and blue field are matched at a high brightness level and then compared 



