538 
CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. FESTING 
Diaphragms inside the box cut off any stray rays of light, and rotating sectors placed 
at A and B regulate the strength of the beams. The room containing the apparatus 
is darkened. The sectors A are closed until no colour is discernible in the mono¬ 
chromatic beam, whilst the intensity of the white beam regulated by the sectors B 
gives the standard of whiteness to which the coloured beam is to be reduced. It is 
worthy of notice that when the white beam is entirely cut off, or made very feeble, 
colour often seems absent from the monochromatic light, but is again perceived when 
the beam is brightened. This is especially the case with the red part of the spectrum. 
The strength of the coloured beam was therefore always reduced to the point that no 
colour was apparent whatever was the strength of the white beam. The aperture of 
the sector A was noted for each colour. The direct measurement of such a feeble 
Fig. 36. 
light would be very difficult, the luminosity was therefore determined in the following 
manner. The box and sectors were removed, and a white screen was placed at the 
same distance from M that S was. The card carrying the slit in the spectrum was 
also removed so that a patch of white light was received on the screen, the luminosity 
of this was measured by direct comparison with an amyl-acetate lamp. The mirror 
l/i was next removed, and the beam then fell on the screen of the original apparatus. 
Its luminosity was then compared with the reference beam. The slit slide being put 
back in the spectrum, the luminosity of the D light was measured against the same 
comparison light. The proportion that the luminosity of the D light bore to the 
