Artificial Lighting 179 



red of one's lips appears dark or greenish black ; and the white 

 of one's skin becomes a ghastly white. White and black, which 

 contain no admixture of colors, remain entirely normal. This 

 is exactly the effect needed in the artificial illumination of offices 

 and factories where there is no necessity for color discrimination. 

 When the rays of the mercury vapor light are analyzed by 

 means of a glass prism, the spectrum (page 180) shows only 

 green, blue, or violet, and faint yellow rays of light. There 

 is no red in the light, which explains why the red color of objects 

 is modified when seen in such light. This raises an interesting 

 question which we shall consider in some detail. 



III. THE COLORS OF OBJECTS 



71. Upon what do the colors of objects depend? Do colors 

 depend upon the nature of the objects alone? Do they de- 

 pend upon the quality or kind of light falling upon them ? Or 

 do they owe their colors both to their own nature and to the 

 quality of light which falls upon them? We can only suggest 

 to you a way to find an answer to the question. 



Common opinion may insist perhaps that the color of an 

 object depends upon the object alone and cite the familiar fact 

 that grass is green and the buttercup yellow, as every one knows. 

 But is grass green and the buttercup yellow in all kinds of light ? 

 One's lips are certainly not red in the greenish light of the mer- 

 cury vapor lamp. 



It is easy to produce lights that do not have all the colors of 

 the rainbow or spectrum, but are entirely lacking in one or 

 more colors. Then we can bring within the range of such 

 light certain objects with colors which we know in daylight and 

 we can observe the effects. One such light may be prepared 

 in the following way. 



Exercise. Mix a tablespoonful of common salt with some de- 

 natured alcohol and put the mixture into a shallow dish, such as 

 a saucer. Have some differently colored papers numbered or with 

 labels on which the names of the colors are written. Darken the 



