DISPERSION. 
and exceptional cases that the light emitted or reflected by 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 (chroma) signifying 
COLOTJE. 
60. The extent of the chromatic aberration produced by a lens 
measured by the interval v e (fig. 31) between the red and violet 
images, is called the dispeesiox 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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