COL O UR-BLINDNESS. 1 09 1 



Hering x has objected to the results of Konig and Brodhun, that they 

 were made with too large a field, and he found that with so small a field 

 that the area stimulated was limited to the macula, matches at one 

 intensity were good for any other, v. Kries and Nagel, 2 using a small field 

 without absolutely accurate fixation, have compared equations with the 

 eye adapted to bright light and to darkness, and have found a very marked 

 difference in the two conditions. With a still smaller field, and with 

 accurate central fixation, it was found that there was a central area of 

 the visual field in which there was no deviation from Newton's law, and 

 a match was good at all intensities. Two dichromats (Nagel and Stark) 

 were able to measure this central region, and found that at 1 metre 

 from the eye its vertical diameter was between 30 and 40 mm., these 

 limits corresponding closely with those of foveal vision. 



The two kinds of red-green blindness. — The features described 

 are common to all cases in which red and green are confused. In 

 respect of certain other features, these cases fall into two groups, 

 which, in accordance with a special theory, have commonly been 

 known as red-blind and green-blind. In the present state of the 

 subject it is better to use terms which do not involve theory, and v. 

 Kries has lately suggested that the names protanopic and deuteranopic 

 shall be substituted for red-blind and green-blind. These names have 

 the disadvantage that they give no clue to the nature of the difference of 

 vision in the two groups. The most obvious difference is in the relative 

 brightness of the red end of the spectrum. To one group it is very 

 dark, or even black (i.e. the spectrum is shortened), while to the other 

 its brightness is approximately as great as to the normal eye, and this 

 difference may be expressed by the terms scoterythrous and photerythrous? 

 In the scoterythrous group the spectrum is more or less shortened at 

 the red end, the amount of shortening being, however, influenced by 

 intensity and by adaptation, especially adaptation to red. Consecpiently, 

 a colour-blind case must always be compared with a case of normal 

 vision under exactly similar conditions. In this group the maximum 

 brightness is shifted towards E, as compared with normal vision. 

 According to Abney, it is about 559 X. In the photerythrous group the 

 maximum brightness is about D, and the spectrum has approximately 

 the same relative brightness and the same red limit as in normal vision. 



The neutral band has been said to be nearer the violet end in the 

 scoterythrous group, but this does not seem to be the case. In all cases 

 it lies between 505 and 492 A, but Konig found that its position varied 

 within these limits, independently of the other differences. According 

 to v. Kries, the differences in the two groups only amount to 4 to 6 X, 

 and this may easily be obscured by the factors mentioned in the last 

 section. 



It is a point of great theoretical importance, whether these two 

 groups are sharply marked off from one another, or whether they are 

 connected by transitional cases. As regards shortening, maximum 

 brightness, and neutral band, there is no definite evidence in favour of a 

 distinct separation without transitional cases ; but in the case of certain 

 other features the evidence appears to be conclusive. 



1 Lotos, Prag, 1885. Bd. vi. S. 142 ; Arch. f.d. gen. Physiol., Bonn, 1893, Bd. xlvii.S. 277. 



" Ztschr. f. Psychol, u. Physiol, d. Hinncsorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 1. 



3 I am indebted to Dr. Verrall of Trinity College, Cambridge, for suggesting these 

 designations. They do not imply that either group see red, but only that the light which 

 is red to the normal eye is relatively dark to one and relatively light to the other group. 



