OPTICAL IMAGES. 
attained, admits of an explanation, which after what has been 
stated above will be easily understood. 
Let an object o o, fig. 35, be placed before a convex lens, c c, 
and let v be its violet, and B its red image, the dispersion being 
Fig. 35. 
consequently v e. Now, let F f be a concave lens, through which 
the rays proceeding from c c will be transmitted. This lens being 
concave, will have the effect of diminishing the convergency of 
the rays, and of throwing both the violet and red images to a 
greater distance ; but it will have a greater effect on the violet 
than on the red rays, the former being more refrangible. Now, 
suppose that the material of which the lens F F is made, be such 
that at a certain distance from it, at v' for example, the quantity 
of dispersion it would produce would be exactly equal to v B. In 
that case it is evident that the extreme images of o o, the violet 
image and the red image would be equally affected in contrary 
directions by the two lenses c c and f f. Bye c, the violet image 
would be brought back, and the red image thrown forward, so as 
to separate them by the distance v e ; but by the lens F F, on the 
contrary, the violet image is thrown forward, and the red driven 
back, in exactly the same degree, so that the two images are made 
to coalesce at b' v'. As to the intermediate images, although they 
do not actually coalesce, their dispersion becomes so insignificant 
as to produce no perceptible chromatic aberration. 
The production of this effect depends on the relative dispersive 
and refractive powers of the material of the two lenses, and on 
their forms. 
This important principle may be further elucidated as follows; 
Let l' 1 / (fig. 36, p. 113) be a diverging lens and let it be 
supposed to receive rays proceeding from a white object which, 
if not intercepted, would produce a real image of the object at 
a point o, within the focal distance of the lens x/ I/. In that case 
the lens L' l', according to what has been explained, will produce 
a series of coloured images of the object at a greater distance 
116 
