332 



Tables 345-347 

 VARIOUS PROPERTIES OF THE EYE 



TABLE 345. — 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 intensities. For eye accommodated to 25 cm, ratio apparent to true pupil, 1.02, for the unaccom- 

 modated eye, 1.14. The pupil size varies considerably with the individual. It is greater with one eye 

 closed; e.g., it was found to be for 0.01 millilambert, 6.7 and T.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 indi- 

 vidual. At the extreme intensities the two values approach each other. The ratio of the extreme pupil 

 openings is about 1/16, whereas the light intensities investigated vary over i,ooo, 000-fold (Blanchard and 

 Reeves, partly unpublished data). 



TABLE 346. — Relative Visibility of Radiation (International Standard — Geneva, 1924) 

 (See Gibson, Tyndall, Bur. Standards Sci. Paper 475, 1923; Judd, Journ. Opt. Soc. Amer., 21, 267, 1931.) 

 This table gives the relation between luminous sensation (light) and radiant energy. Data determined 

 for intensities above Purkinje effect. See Table 339. Ratio of light unit (lumens) to energy unit 

 (watt) at .55/i, 0.00162 (Ives, Coblentz, Kingsbury). 



TABLE 347. — 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. H2O, .34 cm, n = 1.337); M front surface lens (c, id mm); (/) lens (equiv. H2O, .42 cm, n = 1.445); (g) back 

 surface lens (c, 6 mm); (//) vitreous humour (equiv. H2O, 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 2/u) and cones (10 (6 outside fovea) /J. long). Rods are more numerous, 2 to 3 between 

 2 cones, over 3,000,000 cones in eye. Macula lutea, vellow 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 3^. In region of dis- 

 tinct vision (fovea centralis) smallest angle at which two objects are seen separate is 50" to 70" = 3.65 to 5-HM at 

 retina; 50 cones in ioom here; 4^ between centers, 3fi to cone, im 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, .4.11, .031 sec.; .45U, 

 .0:0 sec; .5ju, 015 sec.; .57^, .012 sec.; .68/x, .014 sec.; .76^. .018 sec; for intensity, .06 meter-candle, .028 sec; 1 mc, 

 .020 sec; 6 mc, .014 sec; 100 mc, .010 sec; 142 mc, .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. 



