1892.] on tlie Sensitiveness of the Eye to Light and Colour. 603 



only say that it has in all colour investigations been of such service 

 that any attempt on my part to do without it would have been most 

 disadvantageous. The apparatus enables a patch of what is prac- 

 tically pure monochromatic light of any spectrum colour to be placed 

 upon the screen at once, and an equally large patch of white light 

 alongside it, by means of the beam reflected from the first surface of 

 the first prism. 



It sliould be pointed out that this beam of white light reflected 

 from the first prism of the apparatus, having first passed through 

 the collimator, must of necessity diminish with the intensity of the 

 spectrum, when the collimator slit is closed. 



Fig. 1. 



Extinctiou of Spectrum Colours. 



Having got these patches, the next step is to so enfeeble the 

 light that their colour and then their visible illumination disappear. 



An experiment which well demonstrates loss of colour is made 

 by throwing a feeble white light on one part of the screen, and then 

 in succession patches of red, green, and violet alongside it. The 

 luminosity of the coloured light gradually diminishes till all the 

 colour disappears, the white patch being a comparison for the loss of 

 colour. 



If red, green, and violet patches be placed alongside each other, 



2 s 2 



