On artificial temporary Colour-blindness. 37 



Examination of the Phenomena with a Spectroscope of Wide Dispersion. 



Green blindness was produced by the spectroscopic method already 

 described. When the exposure was complete the slit was closed 

 until the Fraunhofer lines were sharply defined. All sensation to 

 green was lost, the red appearing to meet the blue in the centre of 

 the field. The position of the junction of these two colours could be 

 varied considerably by exposing the eye to strong red or strong 

 blue light, thus showing that the red and blue overlap. But violet 

 light had no effect upon the position of the junction of red with 

 bine. 



Similarly, during blue-blindness, the green and violet were seen to 

 overlap, exposure to green light shifting the junction towards the 

 green and vice versa. Red light had no effect on the position of 

 the junction of green with violet during blue-blindness. 



The phenomenon of flickering* visible between the red and the 

 green of a highly magnified spectrum, is also seen at the junction of 

 red with blue during green-blindness, and of green with violet during 

 blue-blindness, as well as at the junctions of green with blue and of 

 blue with violet under normal conditions. 



The author has succeeded by an exposure of three minutes to 

 light from between H and K in blinding the eye to violet without 

 affecting the blue, the real hue of which is thus seen, unaccompanied 

 by any other colour sensation. 



These experiments lead to the conclusion that no one colour- 

 sensation is related to any other in the sense indicated by Hering. 

 Each may be exhausted without either weakening or strengthening 

 the others. The observed facts are, in the author's opinion, more 

 in accordance with the Young-Helmholtz theory, but they imply the 

 existence of a fourth colour sensation, namely, blue. . 



Examination of the Colour Sensations of 109 Persons. 



The tests employed were Holmgren's wools, supplemented by gela- 

 tine films stained with various colours, Bering's method of coloured 

 shadows, and the author's spectroscopic method, which was applied 

 to seventy normal cases in the following manner. 



Using the large spectroscope referred to, with the slit narrow so as 

 to give a comfortable degree of illumination, the observer selects 

 those portions of the spectrum at which he sees a marked change of 

 hue. He then looks at the red between A and B for thirty seconds, 

 and at a given signal traverses the spectrum rapidly, stopping at the 

 first of these changes. 



Next he looks at the green for thirty seconds before turning to the 



* c Physiol. Soc.' Proc., 1 June, 1897. 



