20 CONCERNING THE RULES AND APPLIANCES 



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 im- 

 probable 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 

 Hsemometer, there are many observers whose sense of colour is 

 in the beginning, or at least after a little practice, so keen that 

 they are able to detect with the greatest exactness the inequality 

 of the colouring m 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., there- 

 fore the difference in the graduation of normal human blood 

 amounts to about 18 per cent, 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 colour in the thickness of the 

 red glass. Together with the ability to distinguish such slight 

 differences in the intensity of the colour, there is combined a real 

 advantage in the use of the Hsemometer. 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 coloured 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 

 colour is the same on both sides of the partition wall, and this 

 point must be in the centre of the partition wall if the increasing 

 variations are alike at both ends. To carry out this arrangement 

 the division of both halves of the coloured circle into three 

 subdivisions (so that there are 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 which these directions for the use of the Haemometer will 

 probably have upon the most of my readers, but that I will also 

 find this impression well founded by the evident incongruity 



