THE CONQUEST OF NATURE 



hands, as referred to a moment ago. In many ways 

 this feature of the light is really a peculiarity rather than 

 a defect, and for practical purposes in work requiring 

 continued eye-strain the absence of the red rays is 

 frequently advantageous. In such close work as 

 that of pen-drawing, for example, some artists find it ad- 

 vantageous to use globes filled with water tinted a faint 

 green color, placed between the lamps and their paper, 

 the effect produced being somewhat the same as that 

 of the mercury- vapor light. For such work the absence 

 of the red rays of the Hewitt light would not be con- 

 sidered a defect; and in workshops and offices where 

 Mr. Hewitt's lamps are used the workmen have become 

 enthusiastic over them. 



On the other hand, the fact that the color-values of 

 objects are so completely changed makes this light 

 objectionable for ordinary use; so much so, in fact, that 

 the inventor was led to take up the problem of intro- 

 ducing red rays in some manner so as to produce a pure 

 white light. He has partly accomplished this by means 

 of pink cloth colored with rhodium thrown around 

 the glass; but this causes a distinct loss of brilliancy. 



The most natural method of introducing the red 

 rays, it would seem, would be to use globes of red 

 glass; but a moment's reflection will show that this 

 would not solve the difficulty. Red glass does not 

 change light waves, but simply suppresses all but the 

 red rays; and since there are no red rays in the mercury- 

 vapor light the result of the red globe would be to sup- 

 press all the light. Obviously, therefore, this apparently 

 simple method does not solve the difficulty; but those 



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