STUDIES IN SPECIAL SENSE PHYSIOLOGY 3G7 



characteristic secondary illumination ; small lines passing through 

 the fixation point exhibit a distinct interruption in the secondary 

 image " (v. Kries, 17 p. 225). 



The great theoretical importance of this failure at the centrum 

 has led to much, at times, acrimonious discussion. Hess ( 20 ) ob- 

 jected that v. Kries' experiments were vitiated by the use of bright 

 objects as fixation marks and too short intervals between successive 

 stimulations. He showed that no interruption of the secondary 

 image of a bright line occurred at the fovea, and obtained with 

 chromatic stimuli similar results, except that for strong red light 

 the secondary was not complementary but homoiochromatic. 

 Hess, accordingly, was of opinion that no central difference exists 

 with regard to the secondary image. 



These objections have been considered by v. Kries in an 

 interesting and lucid article ( 2l ). 



Failure of a central secondary can be demonstrated by rotating 

 a screen with a slit in it before an ordinary projection lantern. 

 The best results are obtained with an arrangement such that the 

 object takes the form of a line J to i broad. If adaptation and 

 physical brightness are so chosen that the secondary is distinctly 

 separated from the primary, the latter's failure over the fovea 

 is clearly demonstrated. Experiments conducted without bright 

 fixation points and with long intervals between the successive stimu- 

 lations gave the same results. V. Kries, however, found that the 

 experiment failed if crossed lines were used or three small fields in 

 a row. This seems to be due to psychological factors, especially 

 the difficulty of concentrating the attention on any particular 

 point. Thus in the line experiments of Hess,. the failure to obtain 

 a central interruption may be referred to an inability to fixate 

 steadily any given point, so that the relations of the moving images 

 are disturbed and a distinction becomes impossible. 



The results of v. Kries have been essentially confirmed by 

 Macdougall ( 19 ), and Hamaker ( 22 ) also failed to obtain foveal 

 secondaries for blue and green stimuli. On the whole, the evidence 

 in favour of v. Kries' view is strong, and we may consider the third 

 point suggesting the peripheral nature of the secondary image 

 its failure at the fovea as established. 



Passing to the tertiary image, the following characteristics 

 have been made out. The hue is best appreciated when one uses 

 red light, and may, in this case, be very distinct. With increasing 



