SELIG HECHT 



117 



It rests on a movable carriage which rides freely along the bottom of 

 the long rectangular blackened box shown in the figure. The carriage 

 is only shghtly narrower than the box, so that a proper and continuous 

 centering of the lamp is insured. The light from the lamp falls on 

 the opal glass, which diffuses it. It then passes through the filter, 



Top 



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Fig. 1. Diagram of apparatus to secure constant light adaptation, and to 

 measure the course of dark adaptation. An explanation is given in the text. 



'and goes, to the eye. The illumination through the red cross may be 

 varied by changing the distance of the lamp from the diffusing plate 

 of opal glass. The intensity of the illumination emitted by the red 

 cross is measured for a given position of the lamp, and the intensity 

 at any other position calculated on the inverse square law. In these 



