224 



Capt. W. de W. Abney. 



[June 21, 



used in making the match being known the dilution of the colour 

 would be determined readily. 



It was found that a slight change in the contrast took place after 

 repeatedly shifting the eyes from the one cube to the other. For in- 

 stance, the contrast caused by green appeared to lose a little of its 

 red hue, degenerating into a brown-yellow. To avoid this, an artifice 

 was employed, which appears to be completely successful. An ordi- 

 nary box stereoscope, with the lenses removed, was mounted on a 

 stand, and in such a position that when the left eye only saw cube 

 No. II, the right eye saw but No. I cube. Thus, the right eye never 

 saw the contrast colour, whilst the left never saw the match. In 

 this way, by alternately changing the direction of the eyes to the two 

 cubes, a match could be readily made. When the match was con- 

 sidered satisfactory, the eyes were directed to a moderately weak 

 white light, and, after a short interval of time, turned to the two 

 cubes, when, if the contrast colour on the one cube and the mixed 

 colours on the other appeared to match accurately, the necessary 

 readings were taken. 



A 



Subsequently it was found more convenient to move the rod 

 placed in the paths of the two beams of the instrument No. II, so 

 that only one shadow appeared, as in fig. 2. In fig. 2 the stripe of 

 white light, A, is shown. It is obvious that the stripe of colour 

 could be equally well isolated. There is no difference in the contrast 

 colours created in the white by this plan, so that only one table of 

 results need be given. 



It will be seen from the table that different and representative 

 parts of the spectrum were used, being the red, yellow, green, blue, 

 and violet, and that in every case the contrast colours provoked 

 in the white could be matched by a single colour of definite wave- 

 length when diluted by white light. If the contrast colour caused 

 by the green were its complementary diluted by white light, it 

 should be by a purple, which requires a mixture of red and blue, 

 whereas it is an orange. The fact as to whether the contrast colour 





