BELL. — TYPES OF ABNORMAL COLOR VISION. 7 



glass, which by adjustment of S', L' and V can be made to give a pretty 

 good match to the normal eye with pure spectral yellow, brought to 

 the same luminosity by the main slit. The patient is then, the spec- 

 trum having been widely displaced by turning D, required to bring 

 it back for a match. A very slight degree of red blindness causes him 

 to match the synthetic yellow with a green, while a plus red or minus 



.1 



W 



*l 



= Ug 



Figure 2. 



green color-abnormality will produce a reddish match. Filter i is 

 then replaced by a synthetic blue-green, produced by cobalt chloride 

 in acetone solution combined with a uranine filter, and the test re- 

 peated. Here a green blind observer would match with blue and a 

 blue blind case with green. This blue-green junction supplements the 

 other in the diagnosis. Finally for further evidence of -f- or — red. 



