198 
MR. LISTER ON THE IMPROVEMENT OF 
means of destroying with the utmost ease both aberrations in a large focal 
pencil, and of thus surmounting what has been hitherto the chief obstacle to 
the perfection of the microscope. And when it is considered that the curves 
of its diminutive object-glasses have required to be at least as exactly propor- 
tioned as those of a large telescope, to give the image of a bright point equally 
sharp and colourless, and that any change made to correct one aberration was 
liable to disturb the other, some idea may be formed of what the amount of 
that obstacle must have been. It will however be evident, that if any object- 
glass is but made achromatic, with its lenses truly worked and cemented so 
that their axes coincide, it may with certainty be connected with another pos- 
sessing the same requisites and of suitable focus, so that the combination shall 
he free from spherical error also in the centre of its field. For this the rays 
have only to be received by the front glass B, from its shorter aplanatic focus f, 
and transmitted in the direction of the longer correct pencil f A of 
the other glass A. It is desirable that the latter pencil should neither 
converge to a very short focus, nor be more than very slightly, if at 
all, divergent ; and a little attention at first to the kind of glass used 
will keep it within this range, the denser flint being suited to the 
glasses of shorter focus and larger angle of aperture. 
The adjustment for the microscope is then perfected, if necessary, 
by slightly varying the distance between the object-glasses ; and 
after that is done, the length of the tube which carries the eye- 
pieces may be altered greatly without disturbing the correction ; 
opposite errors which balance each other being produced by the 
change. 
If the two glasses, which in the diagram are drawn as at some 
distance apart, are brought nearer together, (if the place of A for instance, is 
carried to the dotted figure,) the rays transmitted by B in the direction of the 
longer aplanatic pencil of A, will plainly be derived from some point (z) more 
distant than and lying between the aplanatic foci of B ; therefore (accord- 
ing to what has been stated) this glass, and consequently the combination, will 
then be spherically over-corrected. If on the other hand the distance between 
A and B is increased, the opposite effects are of course produced. 
In combining several glasses together, it is often convenient to transmit an 
