THE MICROSCOPE. 
distance of the object from it, the linear dimensions of the image 
o o will be 10 times greater than those of the object; and if the 
focal length of the eye-glass be \ an inch, the distance of most 
distinct vision being 10 inches, the linear dimensions of o' o' will 
be 20 times those of o o, and therefore 200 times those of the 
object; the linear magnifying power would in that case be 200, 
and consequently the superficial magnifying power 40000. 
It would seem therefore, theoretically, that there would be no 
limit to the magnifying power of such a combination ; practically, 
however, there are circumstances which do impose a limit upon it. 
It must be remembered that the object must always be placed at 
a distance from the object-glass, greater than the focal length 
of the latter, the magnifying power of the object-glass depend¬ 
ing on the number of times this distance is multiplied, to make up 
the distance of the image o o from l l ; if a very great magnifying 
power be required, the latter distance must be a proportionally 
great multiple of the former, and as the eye-glass must be farther 
from the object-glass than the image, the instrument might be 
increased to unmanageable dimensions. 
There is therefore a practical limit to the increase of the ampli¬ 
fying power of the instrument by the increase of the distance of 
the image o o from the object-glass, and consequently it can only 
be augmented by the decrease of the focal length of the object- 
glass, combined with a corresponding decrease of that of the eye¬ 
glass. By such means, the distance of o from L L will be con¬ 
tained a great number of times in 0 L, while the latter has not 
objectionable length, and the distance of the eye-glass from the 
image o o will be contained a great number of times in the 
distance of most distinct vision. 
The eye and object glasses are usually mounted at the distance 
of 10 or 12 inches asunder, adjustments nevertheless being pro¬ 
vided, by which their mutual distance can be varied within 
certain limits. 
6. A convex lens is generally interposed between the object- 
glass and eye-glass, which receiving the rays diverging from the 
former, before they form an image, has the effect of contracting 
the dimensions of the image, and at the same time increasing its 
brightness. The effect of such an intermediate lens will be 
understood by reference to fig. 2, where ff is the intermediate 
lens. If this lens F F were not interposed, the object-glass L L 
would form an image of the object o at o o ; but this image being 
too large to be seen at once with any eye-glass, a certain portion 
of its central part would only be visible. The lens F f, however, 
receiving the rays before they arrive at the image o o, gives them 
increased convergence, and causes them to produce a smaller 
4 
