1892.] on the Sensitiveness of the Eye to Light and Colour. 605 



E is a tube through which the eye looks at S, which is a black 

 screen with a white spot upon it, and which can be illuminated by 

 light coming through the diaphragm D tirst falling on a ground glass 

 which closes the aperture, and reflected on to it by M a mirror. 



The patch of light of any colour being thrown on D, rotating sec- 

 tors, the apertures of which could be opened and closed at pleasure, 

 were placed in the path of the beam, thus enabling the intensity of 



F.G. 3. 



Extinction of the Spijctriim. 



the patch to be diminished. D could bo made of any desired aperture, 

 and thus the illumination of the ground glass would be diminished 

 at pleasure. After keeping the eye in darkness for some time the 

 eye was placed at E when the white spot illuminated by the colour 

 thrown on D was visible, and the sectors closed till the last scintilla 

 of light was extinguished. This was repeated for rays at different 



