374 TITCHENER AND PYLE— ON THE AFTER-IMAGES [July 23, 



(b) Dark- Adaptation. 



Experiment IV.: The Glass Wedge. — Besides furnishing the 

 hght blue wedge of Exp. L, Professor Brashear suppHed us with 

 smaller and more highly colored wedges of claret, red, orange, 

 green and blue glass. With these, or with combinations of them, 

 we proceeded as follows. 



A sheet of ground glass was inserted in the Hering window of 

 a large dark-room : the width of the strip could be regulated at 

 will. Some 2.50 m. before the window was a table, on which stood 

 a large screen of white cardboard. Immediately behind a vertical 

 slit in this screen (3 by 25 mm.) lay a grooved strip of wood, in 

 which the wedge or wedges could be moved. Observations were 

 made in dark-adaptation. The thick end of the wedge was first 

 shown ; it appeared as black or as dark gray. The wedge was then 

 moved along, very slowly: if the observer saw its color, he tapped 

 with a pencil, and the experimenter withdrew it a trifle, to start 

 again after a few seconds. At a given signal, the observer looked 

 away from the slit to the cardboard screen, or to a black surface 

 directly below the screen, and watched the development of the after- 

 image. The regular observers were T, P, G and N ; a few observa- 

 tions were also made by B. 



Owing to the difficulty of procuring the large glass wedge of 

 Exp. I., these dark-room observations were the first taken. And, 

 in bur desire to do justice to Tschermak's method, we spent more 

 time and trouble upon them than we like to recall. The observer's 

 head was fixed securely in a head-rest; the height of the screen 

 was carefully adjusted; generous time was allowed for adaptation; 

 the admission of light was rigorously controlled, beforehand, by the 

 experimenter ; the uniform movement of the wedge was assiduously 

 practised. We were rewarded, however, by the unequivocal charac- 

 ter of the results. Though observation might be continued for 5 

 min. ; though during this period the observer might tap his glimpse 

 of color no less than seven times ; and though in the control experi- 

 ments, with immediate observation of the part of the wedge finally 

 exposed, a good complementary after-image might be obtained in 



