THE SENSITIVENESS OF THE EYE TO LIGHT 



223 



larger prism two adjacent fields appeared in the eye-piece. Since the 

 upper of the two smaller prisms was moveable along a horizontal scale 

 light of the same or slightly different wave length might be viewed in 

 the field. In this way the amount of change necessary to produce a 

 just perceptible but distinct color difference was measured. Values 

 thus obtained were mean values for at least ten observations made by 

 the same person on the same portion of the spectrum. 



Description of Apparatus. 



One arrangement of apparatus was found sufficient for all in\'esti- 

 gations and is essentially that used by Allen * to measure the luminosity 

 of the spectrum. The general arrangement is shown in Fig. 1. The 

 light from an acetylene fiame A after concentration by lenses B and 

 W, passed through the opening C from a light proof chamber M; then 

 through two nicol prisms (E and F) arranged with their principal 





F«a.i 



sections horizontal; thence through the spectrometer G and was 

 finally viewed in a Hilger eye-piece H in which all the light of the 

 spectrum except a narrow central band was cut off by means of adjust- 

 able shutters. The observer was therefore able to subject his eye to a 

 light stimulus of any desired wave-length. A constant gas pressure 

 was taken to indicate a constant luminosity in the source of light. The 

 intensity of the spectrum was controlled by rotating the polariser. 

 The principal plane of the analyser remained horizontal for all investi- 

 gations in order that the displacement of the light waves as they met 

 *Frank Allen: Phil. Mag. 1911, Vol. 21, No. 125, page 604. 



