ERRORS OF APPRECIATION 



579 



driver of a train sees a signal, he says to himself " that is a green signal and 

 therefore my train may proceed." But supposing all the time it were a red 



NC.U.I 

 YtLLQW. \ N.EU.2 



NCU. 6 



FIG. 298. 



The lantern used in practice for the detection of colour blindness. 

 (EDEIDGE GREEN.) 



: 



signal, and that he called it green through colour ignorance, that man 

 is as much a danger to the community as if he in fact were colour-blind. No 

 test can be too searching, and no borderline case should ever be passed ; the 

 risk is too serious. 



Of tests of theoretical importance some have already been described, namely the 

 measurement of the thresholds for light and colour, the colour-mixing apparatus and 

 the flicker method of photometry. There is however another test which is found 

 to give valuable information, namely the spectroscope test of Edridge Green. The 

 instrument consists of a spectroscope to which is fitted two shutters, one of which 

 may be caused to obscure the spectrum from the red end and the other from the violet. 

 The patient commences the test by placing the shutter on the red side at the place 

 where he sees the red begin. The doctor notes this position on the wavelength scale 

 of the shutter. The patient is then told to move the other shutter until it reaches the 

 place where red changes to orange. This wavelength also is noted. The red side 

 shutter is now moved until it occupies the position of the violet side shutter, and the 

 violet side shutter is now moved until a difference in colour at the two sides of the 

 spectral area which is thus isolated is just not able to be seen. The wavelengths are 

 again noted, and the next area measured off, and so on until the violet end of the spec- 

 trum is reached. A person with normal vision will with this instrument map out 

 between 20 and 30 distinct areas. Abnormal vision may be shown in two ways, firstly 

 by the ends of the spectrum being found in abnormal positions, the spectium being 

 shortened at the red or the violet or both, secondly by the isolated areas being too 

 large and too few, and thirdly by wrong names being applied to some of them. The 

 value of the method is considerable because it shows the presence of all three classes 

 of defect, those due to blindness, those cawed by ignorance of colour names, and those 

 in which the appreciation of colour is deficient. 



