THE WAVE THEORY OF LIGHT. 323 



The prismatic spectrum is Newton's discovery 

 of the composition of white light. White light 

 consists of every variety of colour from red to 

 violet. Here, now, we have Newton's prismatic 

 spectrum, produced by a prism. I will illustrate 

 a little in regard to the nature of colour by putting 

 something before the light which is. like coloured 

 glass ; it is coloured gelatin. I will put in a plate 

 of red gelatin which is carefully prepared of 

 chemical materials and see what that will do. 

 Of all the light passing to it from violet to red 

 it only lets through the red and orange, giving 

 a mixed reddish colour. Here is a plate of green 

 gelatin : the green absorbs all the red, giving only 

 green. Here is a plate absorbing something from 

 each portion of the spectrum, taking away a 

 great deal of the violet and giving a yellow or 

 orange appearance to the light. Here is another 

 absorbing the green and all -the.;-. violet, leaving 

 red, orange, and a very little faint green. 



When the spectrum is very carefully produced, 

 far more carefully than Newton knew how to 

 show it, we have a homogeneous spectrum. It 



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