456 ' LECTURE XXXVIII. 



I 



are most conveniently exhibited by means of the shadows of objects placed 

 in coloured ligh't: the shadow appearing of a colour opposite to that of the 

 stronger light, even when it is in reality illuminated by a fainter light of 

 the same colour. It seems that the eye cannot perfectly distinguish the 

 intensity of a colour, either when the light is extremely faint, as that of 

 many of the fixed stars, which Dr. Herschel has found to be strongly 

 coloured, or when the light is excessively vivid ; and that when a consider- 

 able part of the field of vision is occupied by coloured light, it appears to 

 the eye either white, or less coloured than it is in reality : so that when a 

 room is illuminated either by the yellow light of a candle, or by the red 

 light of a fire, a sheet of writing paper still appears to retain its whiteness; 

 and if from the light of the candle we take away some of the abundant 

 yellow light, and leave or substitute a portion actually white, the effect is 

 nearly the same as if we took away the yellow light from white, and sub- 

 stituted the indico which would be left: and we observe accordingly, that 

 in comparison with the light of a candle, the common daylight appears of 

 a purplish hue. (Plate XXX. Fig. 439 • • 441.) 



