THE MICROSCOPE. 
463 
difficulty to be overcome in the construction of such lenses is of 
a twofold character, viz., first, spherical aberration, or the dis- 
tortion caused by the failure of all the rays to meet precisely in 
the focus, owing to the imperfection of the curvature of the lens ; 
and chromatic aberration, which arises from the different degrees 
of refrangibility of the different coloured rays which unite to 
form ordinary white light. The first difficulty is surmounted 
only by more delicate and improved workmanship ; the second, 
by an ingenious combination of different kinds of glass (as crown 
glass and flint glass), which possess different powers of dis- 
persing the coloured rays, — the one correcting the other. For 
simple microscopes this contrivance at once obviated the incon- 
venience arising from the coloured fringes surrounding the 
image, to which the name of chromatic aberration was applied ; 
but the minuteness of the lenses used in compound microscopes 
led even distinguished opticians to believe that these great dif- 
ficulties could never be sufficiently overcome to render such 
compound instruments of any real service. The clearness of 
definition, and the absence of colour observable in even the 
higher powers now in use, afford sufficient proof that these fears 
have vanished before the advance of skill and science. 
A simple microscope, then, consists of either a single lens, or of 
two lenses combined so as to act as one, and fixed upon a stand, 
so that it can readily be adjusted to an object, and at the same 
time kept more steady than it could be by the hand. A frame- 
work to support the object, and a mirror to collect the fight 
upon it, complete the arrangement. Such a microscope as this, 
simple as it really is, is very useful for many purposes, particu- 
larly for the dissection of vegetable structures, or such objects 
as it is desired to tear up or anatomize under assisted vision ; 
and it is to such an instrument as this that the earlier micro- 
scopic observers were indebted for those discoveries which laid 
the foundation of the present vast accumulation of microscopical 
knowledge. 
The compound microscope, being the form now in general 
use, and for the perfection of which the ingenuity of the opti- 
cian and the mechanic has been taxed to the uttermost, is 
that to which we would direct especial attention. Like the 
simple microscope, this instrument may consist of only two 
lenses ; but these, instead of being simply combined to produce 
the effect of one, have totally distinct functions. One of 
these, nearest the object (the objective, or object glass), 
collects the rays from the object and brings them to a focus, 
producing an image, which is scrutinized by the other lens as 
though it were the object itself. This second lens is called the 
eye-glass, and the separation which is obviously necessary be- 
tween these two lenses, and which is equivalent to the focal 
