SECTIONAL TRANSACTIONS.— J. 375 



say that the spectrum is tinged with red at one end and violet at the other, the 

 central portion of the spectrum being colourless ; another that the spectrum consists 

 of two colours, red and violet, with a small colourless interval ; another that the 

 spectrum contains three colours, namely, red, green, and violet, the orange and yellow 

 regions being designated red-green and the blue region green-violet. Another will 

 say that he sees four definite colours, others five or six and a few seven. It will be 

 seen that our colour sensations are very limited, the person having the most acute 

 colour perception only having seven definite colour sensations. We can, therefore, 

 classify the colour perception of individuals as achromic, dichromic, trichromic, 

 tetrachromic, pentachromic, hexachromic, and heptachromic. 



The term ' dichromic ' is applied to those who have only two definite colour 

 sensations and white. When examined with a bright spectrum, they say that they 

 see only two colours there. In the same way the designations trichromic, tetrachromic, 

 pentachromic, hexachromic, and heptachromic, are applied to those who see in the 

 bright spectrum three, four, five, six or seven colours. Those examined behave in 

 every way as if they possessed the number of colour sensations indicated. 



All Dichromics are not equally Colour -Blind. 



A fact that seems to have been generally overlooked is that colour-blindness 

 found in dichromic vision is a defect of hue perception, and that it is this defect of 

 hue perception which causes the characteristic symptoms of colour-blindness. 



The colour perception of the dichromic varies from those who have a colour per- 

 ception bordering on the trichromic to those who are almost totally colour-blind. It 

 is obvious that a man who cannot see the least difference between the colour of the 

 red and that of the green signal on the railway line, except when one is changed to 

 the other, has a colour perception which is more defective than that of the ordinary 

 dichromic. Though I have never found a dichromic who had a hue perception equal 

 to that of the trichromic, I have examined many who possessed a hue perception which 

 was nearly equal. 



Trichromic vision is quite distinct from the so-called anomalous trichromatism, 

 ninety per cent, of the colour-blind agree with the normal equation. 



Abnormalities and defects of light perception may be subdivided as follows : — ■ 



1. Increase or diminution in the visible range of the spectrum. 



2. Defective sensibility for certain wave-lengths. 



3. Increased sensitiveness for certain wave-lengths. 



4. Variations in the maximum of the luminosity curve. 



5. Increase or defects in the power of dark adaptation. 



(a) Very rapid or slow dark adaptation. 



(b) Very complete or imperfect dark adaptation. 



It will be seen that the terms used in previous classifications are quite meaningless ; 

 for instance, a so-called typical red-blind is a dichromic with a considerable shortening 

 of the red end of the spectrum, but this shortening may be associated with normal hue 

 differentiation, and scarcely two cases agree in the amount of shortening. 



Mr. C. A. Mace. — Factors Determining 'Natural' Rates of Mental arid 

 Physical Work. 



The specific questions from which an extended course of investigations has arisen 

 were (1) To what extent is the ratio of 'natural' to maximum rates of working constant ? 

 and (2) Is the natural rate of working more efficient than any prescribed rate ? 



Provisional answers, limited in application, have been obtained. (1) The ratio is 

 far from constant, but at natural rates of working the individual's measure is some- 

 times more constant, and individual differences more marked. Hence it is desirable 

 that more mental tests should be standardised for both rates. (2) The greater 

 efficiency sometimes observed with subjectively preferred rates of working is a transient 

 phenomenon disappearing with practice. In general a law of inverse variation 

 between speed and accuracy appears to hold over a wider range than has been 

 supposed. 



On the more general questions which arose suggestive observations were made in 

 the course of the inquiry. On the basis of these observations further investigations 

 have been opened up which promise to elucidate the distinction between abilities 

 proper and temperamental traits. 



