38 DISPERSION. 



fore continue parallel after refraction. This not being the 

 case, we conclude that the rays composing the incident beam 

 are of different degrees of refr eligibility (the more refrangible 

 rays going to form the upper part of the spectrum, and the 

 less refrangible the lower), and that the elongation of the 

 solar image, and the variety of its colouring, arise from the 

 separation of these rays in their refraction through the 

 prism. 

 * 



(47) It further appears that the rays, which differ in re- 

 frangibUity, likewise differ in colour. The spectrum is red at its 

 lowest or least refracted extremity, violet at its most refracted 

 extremity, and yellow, green, and blue, in the intermediate 

 spaces, these colours passing into one another by impercep- 

 tible gradations. Sir Isaac Newton distinguished seven 

 principal colours in the spectrum, and measured the spaces 

 occupied by each. These colours, arranged in the order of 

 their refrangibility, are red,- orangey yellow, green, blue, indigo, 

 violet ;* of which the yellow and orange are the most lumi- 

 nous, the red and green next in order, and the indigo and 

 violet weakest. 



Any one of these rays may be separated from the rest by 

 transmitting it through a small aperture in a screen which in- 

 tercepts the remainder of the light. The ray thus separated 

 may be examined apart from the rest, and will be found 

 to undergo no dilatation, or change of colour, "by any subse- 

 quent refractions or reflexions. We are, therefore, warranted 



* The imperfection of Newton's classification of colours has been pointed 

 out by Professor Forbes and others. The" indigo ought not to have been dis- 

 tinguished from the blue, the difference to the eye being much less, in kind, 

 than between any other two adjacent colours of the scale. We may, there- 

 fore, better distribute the colours of the spectrum into six, viz., red, orange, 

 yellow, green, blue, and violet, of which the red, yellow, and blue may be re- 

 garded as primary colours, and the orange, green, and violet as sec ondary. 



