DISPERSION. 



and exceptional cases that the light emitted or reflected hy any 

 body is pure homogeneous light. It follows, therefore, from what 

 has been explained above, that as many distinct images of each 

 object will be produced by a lens as there are distinct homogeneous 

 colours which enter into the composition of the light it emits or 

 reflects, and that these several images will be placed at several 

 different distances from the lens corresponding with the different 

 refrangibilities of the different homogeneous lights of which they 

 are composed. 



If different parts of the same object* be differently coloured, 

 different series of images of those parts will necessarily be pro- 

 duced at different distances from the lens, according to their 

 several component colours. 



59. From all this it might be inferred that the optical utility 

 of lenses would be utterly destroyed in the case of all objects save 

 such as would emit or reflect homogeneous light. For if such 

 a multitude of variously coloured images be formed at various 

 distances from the lens, the effect which would be produced upon 

 a card held at any distance whatever, might be supposed to be a 

 confused patch of coloured light, having no perceptible resemblance 

 in form or colour to the object; and such would certainly be the 

 case if the distances of the several images, one from another, 

 were considerable. These distances, however, are so small, that 

 the coloured images are so blended together that the decomposi- 

 tion of their colours appears principally by coloured fringes 

 produced upon their edges, and in general upon the outlines of 

 their parts. Nevertheless, when these false lights and fringes are 

 magnified, as in the compound microscope they always are, by 

 the eye-glass, the general appearance of the object under 

 observation would be so changed as to colour, and so indistinct as 

 to outline, as to be rendered useless for all the purposes of 

 scientific enquiry. 



The indistinctness of the image thus produced, is called chro- 

 matic aberration, from the Greek word xP M ^ a (chroma) signifying 

 COLOUR. 



60. The extent of the chromatic aberration produced by a lens 

 measured by the interval v R (fig. 31) between the red and violet 

 images, is called the DISPERSION of the lens. 



The preceding observations have been applied only to the images 

 produced by a convex lens, but they are equally applicable to 

 concave lenses, taking into account that the images in the case of 

 these last are imaginary. Thus, if a white object be placed before 

 a concave lens, the light issuing from it, after passing through 

 the lens, will proceed as if it had diverged from different objects, 

 leaving the seven colours placed at different distances from the 



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