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Prof. F. Holmgren. 



[Jan. 13, 



other was more or less colour-blind, or that while he was colour-blind 

 with one eye, the other should be imperfectly so, perhaps in so slight 

 a manner that it might be called nearly normal. 



Besides the congenital defect, there exists an acquired one, which 

 does not necessarily affect both eyes at the same time. 



A combination of a normal and an abnormal eye with the same brain 

 is, as we see, not impossible. A few such cases have been noticed in 

 the literature of the subject as curious exceptions, one recently by 

 Professor Becker, in Heidelberg, which is described by him. Such a 

 case is what is wanted : a colour-blind person who can make his con- 

 ception of the different colours subjectively clear for a normal-eyed 

 person, as well as this latter can make the same conception objectively 

 clear to all other people with normal sight. We have thus a bridge 

 between the subjective perception of the colour-blind person and 

 objective scientific research. 



Led by this idea, 1 have during the last three years looked for cases 

 of this one-sided colour-blindness in combination with my statistical 

 researches through Sweden. The difficulties that formerly were so 

 enormous, and which have been removed by my method with skeins of 

 Berlin worsted, were at this trial still greater, from different causes. 

 In Sweden I had previously only found one case, and this (found in the 

 summer of 1879) became unhappily useless through an accident. 



Since I succeeded in finding a practical way of lessening those 

 difficulties, I have, within a comparatively short time (June to October, 

 1880), been fortunate enough to examine two such cases, one of one- 

 sided violet-blindness, and the other (for which I have to thank 

 Professor Hippel, of Giessen), a case of one-sided red-blindness. 



Experience will probably show that such cases are not so rare as we 

 have hitherto thought, and we have every reason to hope for a speedy 

 and perfect solution of the problem in this way ; but the results of 

 these two cases have been so remarkable that I will here give a short 

 description of them. 



The plan, principle, and result were as follows : — 



First the diagnosis of both eyes was carefully made. In both cases 

 there was found a perfect, typical, partial colour-blindness on one of 

 the eyes (the violet-blind on the left, and the red-blind on the right 

 eye) ; the other eye had a weak colour-sense, but still so nearly normal 

 that the principal colours were ascertained with perfect ease. A 

 slight hesitation was only shown in distinguishing the lightest and 

 darkest shades of those colours. Both the cases were thus perfectly 

 fit for the purpose. 



The principle of the trial was exactly this : — To let the normal eye 

 control the perception of the abnormal one, and bring the result into 

 a form that was perfectly plain to other normal- sighted persons. 



A one-sided colour-blind person has through his normal eye a per- 



