13 



seen. On the other hand the red end of the speetrnni will appear 

 red. as soon as it is perceived. By very great intensities tlie colonrs 

 will again loose in saturation '). The magnitude equation found for 

 Osthoff's colours is in accordance with these facts. 



Figure 1. 



') E.g. the wire of the electric lamp behind the darkroom glass, only letting red 

 light through, appears yellow. 



