Survey of the Colour Vision of 1000 Students. 187 



circles represent men and the crosses women. This diagram 

 is a frequency curve of the kind used by statisticians, 

 and the object of the investigation was to find whether this 

 frequency curve was of the type represented by fig. 2 or 

 fig. 3, i. e. whether the colour-blind were merelv the "outliers 



JMg. 



Fiff. 3. 



of a homogeneous population''' or whether they were an 

 independent or independent homogeneous populations of 

 their own. The second alternative is required by the 

 Young-Helmholtz and Hering theories. The conclusion 

 arrived at was that normal colour vision has quite enough 

 " scatter " to include colour-blindness as an outlying portion 

 of itself, but that it would require a very much greater 

 number of observers than 79 to positively disprove the 

 existence of separate maxima for the dichromats and mono- 

 chromais. 



The present investigation is an attempt to get more 

 certainty by using a greater number of observers. At the 

 same time an entirely different experimental arrangement 

 has been employed. 



It was stated on p. 586 of the former paper that one had 

 to trust to the judgment of the observer as to when the 

 difference of colour appeared and that, while nothing occurred 

 to indicate carelessness on the part of the observer, still it 

 would be better if a method could be employed in which the 

 readings of the observer were checked. Consequently 

 attempts were made to devise a new apparatus, and after a 

 little trouble the following very simple and satisfactory 

 arrangement was adopted. 



A low-power microscope was taken, one previously em- 

 ployed in the elementary laboratory course for determining 

 the index of refraction of glass slabs. The focal length of 

 its object-glass was 4*0 cm. and the height of the object- 

 glass above the stage could be determined from a millimetre 

 scale ; when the microscope was focussed its tube moved up 

 and down this scale. The microscope magnified about 

 25 times. On the stage was placed a slide carrying ten test 



