62 



Captain W. cle W. Ahney 



[Feb. 25, 



a lens, L3, on d, a movable ground-glass screen. The rays are collected 

 by a lens, l^, tilted at an angle as shown, to form a white image of the 

 near surface of the second j)rism on p. 



Passing a card with a narrow slit S2, cut in it in front of the 

 spectrum, any colour which I may require can be isolated. The 

 consequence is that, instead of the white patch upon the screen, I 



Fig. 1, 



^ 



Ls 



^ Li 



Colour Photometer. 



have a coloured patch, the colour of which I can alter to any hue 

 lying between the red and the violet. Thus, then, we are able to get 

 a real patch of very appropriately homogeneous light to work with, 

 and it is with these patches of colour that I shall have to deal. Is 

 there any way of measuring the brightness of these patches ? was a 

 question asked by General Festing and myself. After trying various 

 plans, we hit upon the method I shall now show you, and if any one 

 works with it he must become fascinated with it on account of its 

 almost childish simplicity — a simplicity, I may remark, which it took 

 us some months to find out. Placing a rod before the screen, it casts 

 a black shadow surrounded with a coloured background. Now I may 



