£ ON THE ACTION OF TRANSPARENT BODIES 



to have a greater refractive power than the latter. If we again.' 

 take other two prisms of the same substances, having such re- 

 fracting angles, that a ray of light is equally refracted by both, 

 and if we examine the spectrum which each of them affords, 

 by admitting the sun's light into a dark chamber, it will be 

 found that the spectrum formed by the flint-glass is much 

 longer than the spectrum formed by the rock-crystal. The 

 flint-glass is therefore said to have a greater dispersive power 

 than the rock-crystal, or a greater power of separating the ex- 

 treme rays from the mean ray of the spectrum. If we then 

 take other two prisms with their refracting angles of such a 

 magnitude that they produce spectra of equal lengths, it will 

 be found by a particular mode of examination, that the colour- 

 ed spaces have not the same size in the two spectra. The red 

 and green rays will occupy more space, and the blue and vio- 

 let ones less space, in the spectrum formed by the rock-crystal, 

 than in the spectrum formed by the flint-glass. This want of 

 proportionality, or irrationality in the coloured spaces of differ- 

 ent spectra, is not of such a magnitude as to be visible upon a 

 mere examination of the spectra themselves. In order to ob- 

 serve it, we must make the prism of flint-glass refract in op- 

 position to the prism of rock-crystal ; and if we look at the 

 bars of a window through the combined prisms, we shall per- 

 ceive on the side of the bar to which the vertex of the flint- 

 o-lass prism is directed, a fringe of green light, and on the 

 other side of the bar, a purple or wine-coloured fringe. The 

 flint-glass, therefore, acts less powerfully upon the green rays 

 than the rock-crystal, or they are less separated from the red 

 extremity of the spectrum. These uncorrected colours, have 

 been called the secondary spectrum, and form the principal ob- 

 stacle to the perfection of the achromatic telescope. 



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