ON COLOUR PHOTOMETRY. 
545 
filled the prism, and by using a second lens the faint, continuous spectrum so formed 
was cut off. This plan was found, however, to be too complicated, and was abandoned 
for the simpler one of using absorbing media. A combination of “ cobalt blue ” and 
“ signal green ” glass was used for the violet end of the spectrum, and “ stained-red ” 
glass— i.e., glass flashed on one side with copper, and on the other with gold—for the 
red end. 
Fig-. 41. 
Extinction curves of normal eye. 
The continuous line curves show the proportion of the beam from each part of the spectrum which is 
just not visible, the illumination by the beam from D when unreduced being equal to that of one 
amyl-acetate lamp at one foot from a screen. 
The dotted curves show the proportion, supposing that all beams had equal intensity to that of D. 
The luminosity of each beam after passing the medium was determined, a,Iso the 
proportion left when it was reduced so as just to extinguish the light, the product of 
the numbers representing these quantities would evidently represent the absolute 
luminosity at the point of extinction, or, in other words, the proportion left on the 
supposition of a uniform luminosity for all parts of the spectrum. 
Tables III. and IV. give the results of these observations with which the ob¬ 
servations of Table V. are combined. The figures in the third column represent the 
4 A 
MDCCCXCII.—A. 
