80 THE AMEKlCAN MONTHLY [April, 



the Ha^niometer about one-fourth too low, that is in the examination 

 of normal human blood at about 75 per cent. 



Whether such persons can use the Hfemometer to advantage, and to 

 what extent, and under what conditions, are questions to which the 

 preceding experience can give no definite answer, and whose solution 

 remains for future investigation. Still I wish to express, a priori^ the 

 following conjectures : 



In those who are severely suffering with red-blindness whenever their 

 retina are carefully studied and accurately observed, it is found that the 

 same anomaly exists in all. Such cases afflicted with red-blindness 

 manifest a functionary defect of the sense of color. 



I consider the validity of the same course of reduction-quotients for 

 the totality of the red-blind even more probable than the validity of the 

 same quotient for the whole extension of the Htemometer-scale, every 

 graduation made by one who is red-blind in any definite direction upon 

 the Hjemometer-scale, always through this, one quotient should be 

 changed into the corresponding graduation of the normal eye. 



The inability to see red in its proper degree of intensity seems to be 

 a functionary defect of the sense of color, which occurs in all degrees 

 between the normal eye and the total red-blindness. And I am not as 

 yet convinced that in all the cases the defects extend to and spread 

 wave-like or in a constant ratio over those lying within the defect. 



Under such circumstances it seems to me to be highly improbable 

 for red-examiners to have such a common factor of reduction such as 

 we have observed for the total red-blindness may exist. 



In contradistinction to the above-mentioned rare cases of eyes that 

 are not at all able, or only to a certain degree able to use the Hte- 

 mometer, there are many observers whose sense of color is in the be- 

 ginning, or at least after a little practice, so keen that they are able to 

 detect with the greatest exactness the inequality of the coloring in the 

 part of the wedge suddenly made visible through the comparing vessel. 

 Of course the difference in the thickness of the wedge at both ends of a 

 piece in a position of the same visible at the same time is not less than 

 0.9 mm., therefore the difference in the graduation of normal human 

 blood amounts to about 18% of the central thickness of the wedge. 

 Yet it has been said that every observer is not capable of detecting the 

 corresponding variation of the color in the thickness of the red glass. 

 Together with the ability to distinguish such slight differences in the 

 intensitv of the color, there is combined a real advantage in the use 

 of the Hiemometer. Such observers are able in graduating to seek 

 that position of the wedge in which at the end of the partition wall of 

 the comparing vessel the blood half is more deeply colored than the 

 wedge half; at the other end the wedge half appears darker than the 

 blood half. Between these there must of course be a point at which 

 the intensity of color is the same on both sides of the partition wall, 

 and this pomt must be in the centre of the partition wall if the increas- 

 ing variations ai"e alike at both ends. To carry out this arrangement 

 the division of both halves of the colored circle into three subdivisions 

 (so that there arc six in all) by means of two thin black straight lines 

 perpendicular to the dividing line and dividing the latter into three 

 equal parts, is advantageous. 



I believe that I not only anticipate correctly the surprising effect 



