OPTICAL IMAGES. 
lens of crown-glass will have a longer focus than a similar one of 
flint-glass, since the latter has a greater refracting power than 
the former. 
39. SPHERICAL ABERRATION. 
In all that has been stated hitherto, it has been assumed that 
the images produced by lenses are as perfect reproductions of the 
object as is the image produced by a common looking-glass. 
In practice this conclusion requires considerable qualification. 
In the first place, lenses, of whatever material they may be 
formed, though very transparent are not absolutely so, and they 
consequently intercept more or less of the light which falls upon 
them. The thicker they are the greater is the quantity of light 
thus intercepted. Sometimes there is a tendency to intercept 
light of a particular tint of colour. In such cases the brightness 
of the image is not only deteriorated, but it is falsely coloured, 
being most tinged with those colours which the material of the 
lens transmits most freely. 
Although such imperfections cannot be totally removed, they 
may be and have been reduced to so very inconsiderable an 
amount by the proper selection and adaptation of the material of 
which lenses are formed, that they need not be farther noticed 
here. 
The loss of light by reflectors, however highly polished the 
reflecting surface may be, greatly exceeds the amount of light 
intercepted by transparent media. On this, as well as some 
other accounts, refracting have been generally preferred to 
reflecting microscopes. 
40. Although the image of an object produced by a convex 
lens in the manner already described (29), appears at first view 
to be an exact reproduction of the object, it is found, when 
submitted to rigorous examination, to be more or less confused 
and indistinct. This confusion is augmented in proportion as it 
is more magnified, and when it is viewed as in a compound 
microscope, with a simple microscope so as to be still further 
amplified, the confusion becomes so great as to deprive the 
observation of all utility. 
This indistinctness and confusion arises from two causes, one 
depending on the form, and the other on the material of the 
lens. 
That which depends on the form of the lens we shall now 
explain. 
41. If a convex lens be presented to a visible object, the central 
part being covered by a disc of card, leaving uncovered a ring 
of surface at the borders, a distinct, but very faintly illuminated 
102 
