356 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



screen of blue-green or orange paper, illuminated by lamp light 

 passing through an absorbing medium, the fixed illuminant being 

 so chosen that no colour was discernible. The tested lights were 

 arranged in the first series of experiments at an eccentricity of 

 twenty degrees ; in the second, at one of four degrees. In the 

 first set of observations, Stegmann found that, after five to fifteen 

 minutes' dark adaptation, a diminution of 21 to 45 per cent, in 

 orange intensity, and an increase of 23 to 50 per cent, in blue-green 

 intensity were necessary in order to match the fixed standard ; 

 that is to say, below the chromatic threshold, the peripheral 

 responsiveness was increased with respect to the long waved light 

 and diminished with regard to the short-waved light, i.e. a change 

 the exact converse of that associated with the ordinary Purkinje 

 effect. V. Kries concluded that these results depended on the 

 feeble intensity of the stimulus employed, and that with light 

 stronger but still below the chromatic threshold, the results agreed 

 with the Purkinje effect, the discrepancy being therefore a conse- 

 quence of physical intensity ; this view has not, however, been 

 generally accepted. Recently, G. J. Burch has published the 

 results of some interesting experiments on achromatic thresholds ( 9 ). 

 In the first place, Burch performed some qualitative experiments. 

 A Bunsen burner was completely covered by a metal chimney so 

 as to prevent any escape of light while not interfering with ventila- 

 tion. By bringing the flame into contact with the metal chimney 

 the latter could be heated gradually to a point at which it became 

 luminous. If the experiment were performed in a room with 

 windows covered by ordinary blinds that is to say, in a room 

 from which light had not been absolutely excluded the first 

 appearance of light was a pearl -grey tint the achromatic 

 threshold as had been previously stated. But when the room 

 was changed to one without windows and absolutely dark, 

 the first appearance of luminosity was not grey but dull red. 

 On repeating this experiment, after spending a jew minutes 

 in a lighted room, the former grey appearance was once more 

 obtained. 



Burch then made exact measurements, using spectral red 

 and violet, a Nicol prism and paper reflector (Fig. 1). He 

 found that after a prolonged rest in total darkness, a period 

 of two hours being necessary, no achromatic threshold existed 

 for either light. 



