BREWSTERS ANALYSIS OF LIGHT. 



It must therefore be wholly transmitted, partly transmitted, 

 or wholly intercepted. If it be wholly transmitted, no change 

 will be made, therefore, in its colour or intensity ; if it be partly 

 transmitted, its colour will remain the same, but its intensity 

 will be diminished ; if it be wholly intercepted, the space it 

 occupied on the spectrum will be black. But these are not the 

 effects, as Sir David Brewster states, which are observed. He 

 finds, on the other hand, that the coloured spaces on the spec- 

 trum are not merely diminished in intensity, but actually changed 

 in colour. Now, if any space of the spectrum be changed in 

 colour, it follows from what has been stated, that the light trans- 

 mitted must be a constituent of the colour of that space, to which 

 the light intercepted being added, they would reproduce the colour 

 of the spectrum. By such an experiment as this, Sir David Brewster 

 found that the parts of the spectrum occupied by the orange and 

 green lights produced yellow, from which he inferred that the 

 glass intercepted the red, which combined with the yellow pro- 

 duced orange, and the blue, which combined with the yellow 

 produced green. But if the glass have the power of thus inter- 

 cepting the red and blue light, it might be expected that the red 

 and the blue spaces of the spectrum would appear dark. He 

 accordingly found that the light of the middle of the red space was 

 almost entirely absorbed, as was also a considerable part of the 

 blue space. 



From experiments like these, which he made in great number, 

 and under various conditions, Sir David Brewster deduced the 

 conclusion to which we have adverted above. 



He inferred that at a point of the spectrum, red, yellow, and 

 blue light are combined in various proportions, the colour of 

 each part being determined by the proportional intensities of 

 these three colours in the mixture. In the red space, the pro- 

 portions of blue and yellow are exactly those necessary to produce 

 white light, but the red is in excess ; a portion of it combined 

 with the blue and yellow produces a white light, which is red- 

 dened by the surplusage of red. In the same manner, in the 

 yellow space, the proportion of blue and red is that which is 

 proper to white light, but there is a greater than the just propor- 

 tion of yellow. 



A part of this combining with the blue and red produces 

 white light, which is rendered yellow by the surplus. In the same 

 manner exactly, the blue space is shown to consist of a surplusage 

 of blue, combined with the proportion of red and yellow, and the 

 remainder of the blue necessary for whiteness. The other colours 

 of the spectrum, according to Sir David Brewster, are secondary, 

 or the result of combinations of red, yellow, and bluer 



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