268 On the Achromatic Microscope. 
magnify the diameter, with a proper eye-piece, from ninety to one 
hundred and eighty times.. When the length is so adjusted as to 
make it magnify the diameter one hundred times, the vision 18 so 
distinct that objects are rendered visible, which cannot be seen with 
dny single glass lens. as . 
_ » «In constructing achromatic microscopes, it is very difficult to make 
‘them free from spherical aberration in the first attempt, even when 
we know with precision the required radii and thickness of the lens- 
es3 as a small variation in either will materially affect the spherical , 
aberration. It is therefore found expedient not to rely on theoretical 
calculations, but to ascertain by direct experiment, the radii that will 
destroy, most effectually, the color and spherical aberration in glass 
of a given density. When a standard microscope is thus obtained, 
and several similar ones are required, the most practicable meth 
appears to be, to construct them as nearly similar as can easily be 
done, and then change one of the curves of the first lens, until by 
approximation, the spherical aberration be nearly corrected. When 
this is done, the correction may be completed by changing the size of 
the aperture, enlarging it when the aberration of the convex lens is 
greater than that of the concave, and diminishing when less. The 
correction may also be completed, when the aberration of the con- 
sare is less than that of the convex, by grinding the edges of the 
flint lens on a flat hone, so as to bring its centre and the uncemented 
convex lens nearer together. _ No correction could be made by chang- 
ing the aperture, if no secondary aberration of figure existed ; but in 
consequence of the great increase of this aberration with the increase | 
of aperture,* on the one hand,—and the insufficiency of a, great Te 
duction, (especially if the aperture be small,) to correct the spherical 
aberr ation, on the other,—the extent of this mode of correction is 
necessarily limited. It must, therefore, be first nearly corrected by 
changing one or both of the radii of the first lens. As these change 
: I, they will not much affect the secondary color. BA 
: us It appears from calculation, that the longitudinal secondary aberration of figure» 
in microscopes of large apertures, increases nearly as the fifth powers of the aper 
tures; and that when the apertures are very large, it increases in a still greater ratio- 
(yet 
ye 
See 
The tube of the microscope should be made of variable length. © 
Vhen the sextuple object glass is used, the length may be made to ; 
vary from four to six and a half inches. ‘The microscope will then 
te: 
