INTRODUCTION TO OPTICS. 

 Fig. 28. 



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V 



degrees of refrangibility ; in passing through the prism, therefore, they 

 take different directions, according to their susceptibility of refraction. 

 The violet rays deviate most from their original course : they appear at 

 one end of the spectrum A B. Contiguous to the violet are the indigo 

 rays, being those which have somewhat less refrangibility : then follow, in 

 succession, the blue, green, yellow, orange, and, lastly, the red, which are 

 the least refrangible of the coloured rays. The union of these colours, in 

 the proportions in which they appear in the spectrum, produces in us the idea 

 of whiteness. If a card be painted in compartments with these seven 

 colours, and whirled rapidly on a pin, it will appear white. But a more 

 decisive proof of the composition of a white ray is afforded by re-uniting 

 these coloured rays, and forming with them a ray of white light. This 

 can be done by letting the coloured rays, which have been separated by a 

 prism, fall upon a lens, which will make them converge to a focus ; and when 

 thus re-united, they will appear white as they did before refraction. The 

 prism P (Jig. 29) separates a ray of white light into seven coloured rays ; 

 and the lens L L brings them to a focus at F, where they again appear 

 white : thus, by means of a prism and a lens, we can take a ray of white 

 light to pieces, and put it together again. 



Fig. 29. 



This division of a ray of white light into different colours, being caused 

 by the unequal refrangibility of the different coloured rays, must take 

 place, more or less, whenever the ray suffers refraction. Thus the rainbow, 

 which exhibits a series of colours so analogous to those of the spectrum, 

 is formed by the refraction of the sun's rays in their passage through a 



