for the Measurement of Visibility of Objects. 99 



in such expression is called the " per cent, white/ 5 for which 

 the symbol I is used. Thus a colour for which 1=100 per 

 cent, is equivalent to zero saturation and if 1 = per cent., 

 saturation is 100 per cent. 



It has been demonstrated experimentally that any colour 

 can be matched by the mixture, in the proper proportions, 

 of white light with monochromatic spectral light of the 

 proper wave-length. In this way a direct measurement of 

 the fundamental sensation properties of a colour may be 

 made. The hue is specified by the wave-length of mono- 

 chromatic light used (wave-length of the dominant hue). 

 The saturation is specified either as the purity (per cent, 

 hue) or as the impurity (per cent, white), the former value 

 being obtained from the ratio of the intensity of the mono- 

 chromatic to the total intensity (monochromatic plus white) 

 of the mixture, while the latter value (per cent, white) is 

 given by the ratio of the intensity of the white to the total 

 intensity of the mixture. These values are pure numerics. 

 The usual unit used in expressing the wave-length of light 

 is the millimicron, which is equal to '0000001 centimetre 

 and is designated by the symbol /jljjl. 



In the foregoing paragraphs have been defined the various 

 terms that will be used in the following discussion of the 

 subject of visibility. These are summarized briefly in 

 the following table for convenience of reference : — 



Symbol. Quantity. Unit. 



F Luminous Flux. Lumen. 



E Illumination. Foot Candle. 



B Brightness. Lambert. 



R Reflexion Factor. Percent. 



H Hue. Wave-length (/jl/jl). 



S Saturation 



Purity. Per cent. Hue. 



Impurity. Per cent. White. 



Theoretical Analysis of the Visibility Problem. 



In general it may be said that non-luminous objects are 

 visible by virtue of the light reflected from them. However, 

 any particular object in the field of vision becomes visible 

 as such only by contrast with its surroundings — that is, 

 when the light emanating from that object (either by re- 

 flexion or emission) differs in some respect from the light 



H 2 



