COMPOUND OBJECT-PIECES, 
pendently of other objections ; great powers, therefore, have been 
obtained by combining several 
achromatic lenses in the same 
object-piece, so that the rays 
proceeding from the object are 
successively refracted by each 
of them, and the image sub¬ 
mitted to the eye-glass is the 
result of the whole. 
The optical effect of such a 
combination will be more 
clearly understood by refer¬ 
ence to fig. 11, where l l, l' l', 
and l" l", represent a com¬ 
bination of three achromatic 
object-glasses. Let o o be 
the object, placed a little 
within the focus f of the lens 
L L. The image of o o, pro¬ 
duced by L L, would then be 
an imaginary one in the posi¬ 
tion 11 ; (see Tract on “ Optical 
Images,” 35, et seq t ). After 
passing through L l, the rays, 
therefore, fall upon i/ L', as if 
they diverged from the several 
points of the image 11, which 
may, therefore, be considered 
as an object placed before the 
lens i/ l\ Let f be the focus 
of l' i/; the image of 11 pro¬ 
duced by i/ l' will therefore be 
imaginary, and will be at P i'; 
the rays after passing through 
l' l' will fall upon l" l", as if 
they diverged from the several 
points of i' i\ This image i' i', 
therefore, may he considered 
as an object placed before the 
lens l" l". Let/" be the focus 
of this lens ; the image of i' i' 
produced by l" l" will then be 
l" i", and will be real; this will 
then, in fact, be the image 
transmitted to the eye-piece. 
21 
