BELL. — TYPES OF ABNORMAL COLOR VISION. 9 



tinguished from case (2) by the end points, and so on, assuming that 

 the abnormalities are not vanishingly small. 



The red end point gives fairly definite information but the blue end 

 point is less satisfactory. It is so much modified at times by varia- 

 tion in pigmentation of the macula and by yellowing of the lens as to be 

 somewhat confusing. In the old the lens may be so altered in color 

 that the solar H and K lines cannot be seen. As to pigmentation a case 

 is cited by Abney (Researches in Colour Vision, p. 349) in which much 

 of the blue end of the spectrum was absorbed. Such cases are sepa- 

 rated from a genuine — B case by their giving, with R and G normal, 

 a normal blue-green junction. 



It is pertinent to inquire in how far these twelve binary variations 

 actually occur. The ordinary cases of color blindness reported have 

 not been so tested as to show them easily. The writer has never noted 

 them personally in congenital color blindness, but one (7) is typical of 

 color fatigue due to the mercury-arc and may easily be detected, as 

 found by Williams and the writer (Electrical World, Sept. 2, 1911). 

 A case reported by Edredge-Green (Colour Blindness, p. 154, F. A.) 

 probably belongs to this type, since with a considerably shortened red 

 spectrum his red-green junction was toward the green and he classified 

 violet, blue, and bluegreen together. 



Type (8) probably corresponds to a case described by Burch (Phil. 

 Trans., 199, B, p. 250, XVII). 



Of the next group there is much difficulty in obtaining definite in- 

 formation since for example R,+G,4-B and the ordinary — R, G, B 

 could be distinguished only with some difficulty and would certainly 

 escape ordinary tests. Any case in which some of the ordinary mis- 

 takes of the red blind are made with the confusion colors, while the 

 ordinary end points are retained should be looked into carefully. A 

 plus sensation will rarely be detected by the end points unless the 

 abnormality is very marked. Burch's fatigue tests (Phil. Trans., 191, 

 B, p. 1 et seq.) used to supplement the junction and end point tests 

 probably will prove to give the clearest diagnosis of this group. Lu- 

 minosity tests might be useful since there is evident increase in general 

 sensibility but on account of difficulties due to adaptation and the 

 uncertainty of readings, the experiments are troublesome unless with 

 experienced observers. 



The next group (13) to (18) is easier to deal with and more examples 

 may be found in the literature. (13) is well shown by Burch (Phil. 

 Trans., 199, B, p. 240, VII) in a case of marked but not complete red- 

 blindness with slightly hypernormal green. Type (14) is also clearly 



