Monthly Microscopical l 
Journal, iVlarcli 1, 1869. J 
for the Microscope. 
171 
additional power. If the front of an ^th. or -n^th is tested alone, it 
will be found to magnify very nearly as much as when the other 
lenses are replaced. 
The single front has the advantages of facility of construction, 
and a command of any required extent of aperture ; and enables 
object-glasses of higher power to be made than would otherwise be 
practicable. For example, the radius of the front lens of a -^th is 
-i-4-oth of an inch, and the diameter y^yth. The difficulty, if not 
impossibility, of constructing a triple of such almost invisible 
atoms of glass may be imagined. 
In May, 1856, I made the first ^V^^^i with a single front lens of 
th in diameter; 1 am doubtful whether a triple front could be 
made even of this size, with any positive certainty of accurate work- 
manship. 
From an |th and upwards, perfect correction may be secured 
with a single front. It is, however, barely possible to make a 
good ^th with this form, and in a ^-inch it fails altogether ; there 
is a kind of secondary spectrum that cannot be got rid of. It is not 
easy to define all the reasons why it should succeed perfectly with 
the highest powers, and the correction be imperfect with the lower 
ones named. With smaller aperturcFi the errors of spherical 
aberration cannot be so well corrected by giving thickness to the 
front lens ; and as there is considerable distance between this and 
the middle, the coloured rays from the uncorrected front are so far 
separated, that any corrective action of the back systems is incapable 
of recombining them. When an object-glass is spherically under- 
corrected, the focus of the central rays is longer than that of the 
marginal ones, as in a single lens. If all the rays are brought to 
one point, the interposition of a plate of parallel glass projects the 
outside rays to a greater distance than the central ones, and produces 
a similar effect to a concave lens, or that of over-correction ; and it 
is for this reason that a certain thickness of glass before, or in the 
substance of the front lens has such a remarkable corrective power, 
which is most appreciable with a very large aperture. Where this 
is comparatively small, as in a -J-inch, the influence of a thick front 
does not appear to be sufficient to enable the final correction to be 
obtained by this means alone. The anterior lens must therefore 
either be partly achromatized, or made of a glass of higher refractive 
and less dispersive power than any at present known. 
It is well known that, in a doublet consisting of two single 
plano-convex lenses, both the spherical and chromatic aberrations are 
considerably less than in a single lens of the same magnifying 
power. I have for this reason proposed to construct the higher 
powers with two single lenses in front, of equal radius, as shown by 
