1912.] Negative After-Images and Successive Contrast. 437 



17. When the same was done after fatiguing with blue light the green of 

 the spectrum also changed, becoming a yellower green, and the image seen 

 afterwards on a fresh part of the screen was an orange band with the after- 

 image of the spectrum above and below it. 



18. On fatiguing with red light X.640 the blue-green after-image was seen 

 over all the spectrum with the exception of the yellow, orange, and brightest 

 red ; the after-image on a fresh part of the screen appeared as a blue-green 

 band which was cut into two by the after-image of the spectrum corre- 

 sponding to the yellow region. When this experiment was repeated, the 

 spectrum being viewed for a little longer than before, it was found that the 

 blue-green band was cut through by the whole of the spectrum after-image, 

 but it was noticed that. the after-image of the spectrum faded away the 

 faster of the two, and then the blue-green band ran continuously across. 



19. The eye was then fatigued by red on the screen, using the red filter ; the 

 eye was then removed to the eyepiece of the spectroscope, the green-blue 

 image ran across superimposed upon the image whatever part of the spectrum 

 was selected. 



20. The eye was then fatigued by different regions in the Edridge-Green 

 spectrometer. A monochromatic region was selected and the eye fatigued in 

 a vertical position {i.e. one eye over the other) for 20 seconds. On assuming 

 the normal position the after-image was seen to cross the region. 



No matter what portion of the spectrum was selected the after-image 

 where it crossed the spectral band was seen as a grey square. 



Conclusions. 



The first point which was evident was the very great importance of the 

 intensity of the light which was used, especially in relation to the reacting 

 light. In this respect the results are different from those in which the whole 

 eye is fatigued by a very intense light, as in the experiments of Burch,* or 

 by comparatively weak light acting for a prolonged period upon the whole 

 eye, as in the experiments of Edridge-Green and Devereux Marshall.f As 

 the eye was kept rigidly fixed during the fatiguing process, a very clear cut 

 negative after-image was produced which, when thrown on the screen spectrum, 

 enabled close comparison to be made with adjacent parts. The stability of the 

 after-image was remarkable ; it did not change colour, and was not influenced 

 by subsequent light falling on the retina when this was not of too great 

 intensity. The after-image was in every case darker than any dark object 

 on which it was projected. If the portion of brain having the function of 



* ' Phil. Trans.,' 1899, B, p. 1. 



t 'Trans. Ophth. Soc.,' 1909, p. 211. 



