THE MICROSCOPE. 
Images.’ ? It follows from what is there stated, that all confusion 
produced by this cause, can be removed by substituting for simple 
convex lenses, compound ones, consisting of a 
double convex lens of crown-glass c c', fig. 4, 
cemented to a plano-concave lens of flint glass. 
The image produced by such a combination, 
will be distinct and free from colour, provided 
that certain conditions be observed in the curva¬ 
tures given to its component lenses. 
13. Assuming then, that by such combinations 
the image presented to the eye-glass is a faith¬ 
ful reproduction of the object, in its proper colours, 
perfectly distinct in all its lineaments, and suf¬ 
ficiently amplified, there is still one remaining 
condition for distinct vision, which is, that this 
image should be sufficiently bright. It will, 
therefore, be necessary here, to examine the con¬ 
ditions on which its brightness, or illumination, 
depends. 
In the first place it is very evident that, other things being the 
same, the illumination of the image will be proportionate to that 
of the object, and in the inverse proportion of its superficial 
amplification ; for the light which is transmitted from the object, 
being diffused over the surface of the image, will be necessarily 
more feeble as the superficial magnitude of the image is greater. 
The higher the magnifying power used, therefore, the greater is 
the necessity that the object should be intensely illuminated. 
But the brightness of the image depends not only on the in¬ 
tensity of the illumination of the object, but also on the proportion 
of the light emanating from each point of the object, which arrives 
at the corresponding point of the image; and this, as we shall now 
show, will depend conjointly on the linear opening, or available 
diameter of the object-glass, and the- distance of the object from it. 
To make this more plain, let o o', fig. 5, be the object, and L l' 
the object-glass. We are to consider that each point of the object 
is a centre, from which rays emanate towards 
the object-glass ; thus, for example, rays 
issuing from the point c, form a cone, of which 
the object-glass is the base, and of which c is 
the vertex; supposing all these rays to pass 
through the object-glass, and to be refracted 
by it, they will converge to the point of the 
image which corresponds to c. 
In the same manner, the rays which diverge 
from any other point, such as o, likewise form a cone, of which 
10 
Fig. 4. 
F' C 
