604 



Cajjtam W. de W. Ahney 



[May 6, 



and they are bedimmed in brightness together, it will be noticed that 

 the red disappears first, then the green, and then the violet ;_ or I may 

 take a red and green patch overlapping, which when mixed form 

 orange, and extinguish the colour: the slit allowing red light to lali 

 on the screen may be absolutely closed, and no alteration m the 

 appearance of the patch is found to occur. This shows, 1 think 

 that when all colour is gone from a once brilliant colour, a sort ot 

 steel-grey remains behind, and that red fails to show any luminosity 

 when the green still retains its colour. ,, n-^v * 



The measurement of the extinction of colour from the different 

 parts of the spectrum was made on these principles. A box, similar 



FiG. 2. 



Extinction Box. 



to Fig. 2, was prepared, but having two apertures one at each side. 

 Through one the coloured ray was reflected, and through the other a 

 white beam of light to a white screen. Both beams were diminished, 

 and when the white and coloured patches appeared the same hue, the 

 amount of illumination was calculated. Fig. 1 shows graphically the 

 reduction of illumination, when the D liglit of the spectrum is the 

 same intensity as one amyl-acetate lamp at one foot irom the screen. 

 To measure the extinction of light, a box was made as m the diagram, 

 closed at each end, but having two apertures as shown, l^ig. Z :— 



