486 DIFFERENT REFRANGIBILITY OF RAYS. 



refraction in the same manner as they were disposed 

 originally, and would produce a white spot. But the 

 violet rays are more refracted than the green rays, the 

 green rays more than the red; hence, if the refracting 

 edge of the prism is turned downwards, as has been 

 assumed in the previous experiment, the violet spot 

 produced by violet rays will appear somewhat higher 

 than the green spot produced by green rays, and the 

 green again higher than the red. The spectrum is 

 nothing else but a succession of variously coloured 

 spots or bands of light, each representing a distinct 

 and definite colour. A single colour, however, is seen 

 only at the ends of the spectrum at the lower end 

 red, and at the upper end violet : in the whole inter- 

 mediate space the bands overlap one another partly, and 

 each portion of the spectrum presents a mixture of 

 several adjoining differently coloured rays. 



When the water prism with small refracting angle 

 is used, the dispersion is so inconsiderable (that is, the 

 coloured components of white light differ so little 

 in the amount of deviation which they undergo) that 

 the spots illuminated by each colour almost coincide, 

 the refraction of the violet light being very little 

 greater than that of the red ; hence only a very narrow 

 violet band is seen on one side of the bright spot pro- 

 duced by the light which has passed through the prism, 

 only a very narrow red one on the other side. For the 

 most part, the different colours fall upon the same 

 place, and therefore produce again white light in by 

 far the greater portion of the spectrum. It is obvious 

 that the red band will be above and the violet below, 

 if, as in the arrangement shown in fig. 262, the re- 



