OPTICAL IMAGES. 
from the lens L l. In that case, the distance of the red image R', 
from l' l', will be greater than that of the red image R from L L, 
and in like manner the distances of all the intermediate images 
of o" o' from l' r will be greater than those of the corresponding 
images from L L. 
Thus the coloured images of o‘ o' produced by L l- will be 
spread over a greater space than those of o o produced by L L. 
The dispersion of the latter is therefore greater than the dispersion 
of the former. 
With the same amount of refraction, therefore, the lens L' 1 / 
produces more dispersion than the lens L L. 
If we suppose the convexity of the lens l l to be increased, the 
refraction will be increased, the image v will be produced at a 
less distance from it, and at the same time the dispersion v R will 
be increased. The convexity, as shown at i/' l" (fig. 34), may be 
so much increased, that the dispersion v" r" shall be equal 
to v' R-. 
Thus it appears that a diamond lens, which would have a dis¬ 
persion equal to that of a glass lens, would have a much greater 
refraction, and would produce the image of the same object much 
closer to it. In a word, the focal length of a diamond lens having 
the same dispersion as a glass lens, would be much shorter than 
the focal length of the latter; or, what is the same, with an equal 
focal length, the diamond lens would have a less dispersion. 
64. It appears, therefore, in general, that lenses made of 
different transparent substances will have, under like conditions, 
different dispersions. The dispersive powers of any two 
transparent media, will be measured by the dispersions which 
lenses of the same focal length made from them would produce. 
The actual dispersion produced by a lens must not be con¬ 
founded with the dispersive power of the material of which 
the lens is formed. 
The actual dispersion produced by a lens of a given material, 
varies with its focal length, and with the distance of the object 
from it, so that with the same lens there may be many different 
quantities of dispersion, and the quantity will also be different 
with different lenses of the same material. But the dispersive 
power depends on the material alone, and is altogether independent 
of the form of the lens, its focal length, or the position of the 
object relatively to it. It will be most important that this dis¬ 
tinction should be understood and remembered. 
65. It might be imagined that the dispersive power would 
necessarily increase with the refractive power of the transparent 
body. On comparing together the optical effects of different 
media, no such correspondence is however found to prevail; on 
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