41)1' DP WILLIAM POLE ON THE PRESENT STATE OF 



letter T, a full discussion of the experiments, and it will suffice to present here a sum- 

 mary of the inferences which may be drawn from them. 



1 . The} 7 present a series of variations exactly of the kind met with in the observations 

 of dichromic cases, namely, variations of the colour-strength of the impressions made by 

 the rays belonging to the normal red and green, when compared with the maximum warm 

 sensation. 



2. The two extremes correspond to the two varieties formerly called " green-blindness " 

 and " red-blindness ; " — my own case being a typical one of the former, Mr Parry's of 

 the latter. 



3. The others form intermediate gradations between these ; and the nature of the 

 series gives every reason to believe that the different cases vary merely in degree, not in 

 kind. 



4. The variations in the chromic strength of the red rays are accompanied by cor- 

 responding variations in the chromic strength of the green rays, but in opposite direc- 

 tions ; — i.e., as the strength of the red increases, that of the green decreases, and vice 

 versa. And the variations of both are only in the luminosity, not in the saturation. 



5. These facts are perfectly illustrated by slight variations in the position of the 

 intensity-curve of the yellow sensation along the line of wave-lengths. It will be seen 

 that the variations of the red and green ray impressions may be represented by the vary- 

 ing lengths of the ordinates due to the variable position of the curves. And it is thus 

 shown that a given sensation may be excited by waves slightly differing in length for 

 different individuals; — or, in other words, that the same wave-length may produce a 

 different degree of excitation in different persons. 



6. Although the more important and systematic variations occur in the red and the 

 green rays, the other elements generally show some tendency to irregularity in different 

 persons. 



7. There is no evidence tending to suggest different groups in classification, or any 

 fundamental differences in the cause. 



There is some reason to believe that the difference of position of the yellow curve 

 may make some change in its colour-sensation. It is generally assumed that the sen- 

 sation given by a retina-element excitable over a long range, corresponds with that 

 in the spectrum due to the place of its maximum intensity ; but whether there is 

 sufficient ground for this supposition I do not know. If this is so, the sensation for the 

 extreme left, or " green-blind " (according to Konig), should be about 575, or pure yellow ; 

 while that of the extreme right, or "red-blind," should be 550, or a yellow green. 



Then it is possible, and it is generally assumed, that the shifting of the " red-blind ' 

 curve to the right will cause the neutral point, where the yellow curve intersects the 

 blue one, to be removed nearer to the blue. On this question, also, there is a scarcity of 

 information, owing to the well-known difficulty of determining exactly where the neutral 

 point lies. 



M. Van der Weyde and Messrs Konig and Dieterici make the blue curve identical 



