116 THE NATURE OF FOVEAL DARK ADAPTATION 



In order to confine the experiments to central vision, I utilized 

 its characteristic perception of form and color. The object to be 

 looked at was a red cross. The form was secured by a cross-shaped 

 opening of such size that when looked at directly its image fell well 

 within the rod-free area of the macula. The color was secured by a 

 red filter. The measurements were determinations of the minimum 

 illumination at which the subject could clearly see the object both 

 as a cross and as a red one. 



2. The essential parts of the assembled apparatus are given in 

 Fig. 1, which shows three views of the arrangements. The screen for 

 light adaptation is marked N. It is a closely woven linen screen 

 similar to those used for white window shades. It is illuminated from 

 behind by two Mazda lamps, L, 150 watts and 115 volts, backed by 

 polished reflectors, and set 60 cm. from the screen. The switch for 

 turning the lights on and off is in the hands of the experimenter. The 

 subject looks at the region of the screen lying between the two lights; 

 that is, between the two dashed circles in the front view of Fig. 1 . The 

 brightness of this field is 90 millilamberts, which is equivalent to an 

 illumination of 0.028 candles per square centimeter of screen. 



The construction of the apparatus for measuring the minimum 

 intensity at which the red cross becomes visible is shown in partial 

 section in the side view of Fig. 1. At C there is a sheet of opal glass. 

 In contact with it, on the side facing the observer, is a Wratten Filter 

 No. 70. This limits the transmission to the rays beyond 650 fxp., and 

 thus insures the participation of the cones only. In front of the filter 

 there is the opening already referred to. It is a cross-shaped opening 

 in an opaque piece of cardboard. It is 24 mm. in diameter, each arm 

 of the cross being 8 X8 mm. The area of the cross is thus 320 sq.mm. 

 The observer's eyes are placed next to the opening in the viewing box, 

 above P. The appearance of this opening is seen in the front view. 

 The distance from the eye to the cross is 55 cm. The cross therefore 

 gives an image 0.65 mm. in diameter, which is about two-thirds the 

 diameter of the rod-free area. 



The illumination of the red cross is secured by means of a small 

 microscope lamp situated in a lamp house, and properly surrounded by 

 black screens. It is a 15 watt, 115 volt, concentrated filament, 

 Mazda lamp, and may be considered as a point source for our purposes. 



