DARK ADAPTATION AND NIGHT VISION 



equally sensitive to radiant energy from 

 different parts of the spectrum. 



Although the data cited above are very 

 detailed and informative, they were, un- 

 fortunately, obtained on only one subject. 

 Similar data by Sloan (84) on 101 subjects, 

 ranging in age from 14 to 70 years, are shown 

 in Fig. 4. These measurements were made 

 with a 1°, white test Hght. The solid line in 

 Fig. 4 represents the average threshold 



that Wentworth's eye was atypical may 

 account for the discrepancies. In any case, 

 one thing is clear: the dark-adapted eye is 

 most sensitive at some considerable distance 

 from the fovea. It is interesting to note, 

 incidentally, that these sensitivity curves 

 parallel fairly closely the rod density curves 

 shown in Fig. 2. The eye appears to be 

 most sensitive in those regions where the 

 rods are densest. 



8.0 



NASAL RETINA 



FOVEA TEMPORAL RETINA 



Fig. 4. Luminance of the just perceptible light at various retinal locations. The solid line is the 

 average curve for 101 subjects; the dotted lines enclose the measurements of 95 percent of the subjects. 

 (Data from Sloan, 81) 



values for the 101 subjects. The dotted 

 lines are the average values plus and minus 

 two standard deviations; they thus enclose 

 the measurements of 95 percent of the 

 subjects tested. This study shows the most 

 sensitive part of the retina to be somewhat 

 closer to the fovea than is indicated by 

 Wentworth's data. It is not clear why 

 these two sets of data do not agree more 

 closely. Differences in the colors of the 

 test lights used, differences in the units of 

 measurement employed, or the possibility 



Sensitivity of the Cones and Rods to 

 Radiant Energy. The data by Went worth 

 cited above show very clearly that the eye is 

 not equally sensitive to radiant energy from 

 different parts of the spectrum. This 

 characteristic of the eye has now been 

 thoroughly studied and is usually quantified 

 in the form of photopic and scotopic relative 

 luminosity curves as illustrated in Fig. 5. 

 The scotopic relative luminosity curve in 

 Fig. 5 is an average curve for 48 subjects 

 studied by Hecht and Williams (39). 



