STUDIES IX SPECIAL SENSE PHYSIOLOGY 357 



Adaptation Values (Burch). 

 A. During the period of increasing adaptation. 



B. After spending two hours in the dark room. 



D 



Vff 



Fio. 1. Burch's apparatus for the study of " achromatic'thresholds." 



A. An electric lamp, 1G candle-power. B. Paper reflector. 



:U1 of dark room. D. Stand with polarising prism (E). 



I >I>ectroscope with double image prism (O) over eye-piece. 



There is n stop with two slits (allowing red and blue-violet light to pass through) in the 

 eye-piece. By rotation of the polarising prism the relative Intensities of the red and blue-violet 

 lights can be varied. 



