2^8 TABLES 296-298. 



THE EYE AND RADIATION. 



TABLE 296. Apparent Diameter of Pupil and Flux Density at Retina. 



Flashlight measures of the pupil (both eyes open) viewed through the eye lens and adapted to various field intensi- 

 ties. For eye accommodated to 25 cm, ratio apparent to true pupil, 1.02, for the unaccommodated eye, 1.14. The 

 pupil size varies considerably with the individual. It is greater with one eye dosed; e.g., it was found to be for o.oi 

 miUilambert, 6.7 and 7.2 mm; for 0.6 ml, 5.3 and 6.5; for 6.3 ml, 4.1 and 5.7; for 12.6 ml, 4.1 and 5.7 mm for both 

 and one eye open respectively for a certain individual. At the extreme intensities the two values approach each other. 

 The ratio of the extreme pupil openings is about A, whereas the light intensities investigated vary over i ,ooo,ooo-fold. 

 (Blanchard and Reeves, partly unpublished data.) 



TABLE 297. Relative Visibility of Radiation. 



This table gives the relation between luminous sensation (light) and radiant energy. The results of two methods 

 are given: one from measures of the direct equality of brightness, which some consider the true method, as more direct, 

 but criticized because of the difficulty of judging heterochromatic light (Hyde, Forsythe, Cady, A. J. 48, 87, 1918, 29 

 observers); the other (Coblentz, Emerson, Bui. Bureau of Standards, 14, 219, 1917, 130 observers) depends on the 

 disappearance of flicker when two lights of different color and intensity are alternated rapidly. Color has a lower 

 critical frequency than brightness and disappears first. Data determined for intensities above Purkinje effect. See 

 Table 290. Ratio of light unit Oumen) to energy unit (watt) at 0.5511, 0.00162 (Ives, Coblentz, Kingsbury). 



TABLE 298. Miscellaneous Eye Data. 



Light passing to the retina traverses in succession (a) front surface of the cornea (curvature, 7.9 mm); (b) cornea 

 (equivalent water path for energy absorption, .06 cm); (c.) back surface cornea|(curv., 7.9 mm); (d) aqueous humour 

 (equiv. HjO, .34 cm, n = 1.337); M front surface lens (c, 10 mm); (/ ) lens (equiv. HjO, .42 cm, n 1.445); (s) back 

 surface lens (c., 6mm); (h) vitreous humour (equiv. HjO, 1.46 cm, n = 1.337). An equivalent simple lens has its 

 principal point 2.34 mm behind (a), nodal point 0.48 mm in front of (g), posterior principal focus 22.73 mm behind 

 (a), anterior principal focus 12.83 mm. in front of (a), curvature, 5.125 mm. At the rear surface of the retina (.15 mm 

 thick) are the rods (30 X 2ju) and cones (10 (6 outside fovea) fj, long). Rods are more numerous, 2 to 3 between 

 2 cones, over 3,000,000 cones in eye. Macula lutea, yellow spot, on temporal side, 4 mm from center of retina, long axis 

 2 mm. Central depression, fovea centralis, .3 mm diameter, 7000 cones alone present, 6 X 2 or 3ju. In region of dis- 

 tinct vision (fovea centralis) smallest angle at which two objects are seen separate is 50" to 70" = 5.65 to 5-I4M at 

 retina; 50 cones in 100/1 here; 4/1 between centers, 3^1 to cone, i/x to interval. Distance apart for separation greater 

 as depart from fovea. No vision in blind spot, nasal side, 2.5 mm from center of eye, 15 mm in diam. 



Persistence of vision as related to color (Allen, Phys. Rev. n, 257, 1900) and intensity (Porter, Pr. Roy. Soc. 70, 

 313, 1912) is measured by increasing speed of rotating sector until flicker disappears: for color, .4/1, .031 sec.; .45**, 

 .020 sec.; .SM, 015 sec.; .57^1, .012 sec.; .68/Lt, .014 sec.; .76^1, .018 sec.; for intensity, .06 meter-candle, .028 sec.; i me, 

 .020 sec.; 6 me, .014 sec.; 100 me, .010 sec; 142 me., .007 sec. 



Sensibility to small differences in color has two pronounced maxima (in yellow and green) and two slight ones 

 (extreme blue, extreme red). The sensibility to small differences in intensity is nearly independent of the intensity 

 (Fechner's law) as indicated by the following data due to Konig: 



SMITHSONIAN TABLES. 



