194 
MR. LISTER ON THE IMPROVEMENT OF 
which is considerable at the numerous surfaces of a combination. I have 
thought the clearness of the field and brightness of the picture evidently 
increased by doing this ; it prevents any dewiness or vegetation from forming 
on the inner surfaces ; and I see no disadvantage to be anticipated from it, if 
they are of identical curves and pressed closely together, and the cementing 
medium permanently homogeneous. 
These two conditions then, that the flint lens shall be plano-concave, and that 
it shall be joined by some cement to the convex, seem desirable to be taken as 
a basis for the microscopic object-glass, provided they can be reconciled with 
the destruction of the spherical and chromatic aberrations of a large pencil. 
Now in every such glass that has been tried by me, which has had its cor- 
recting lens of either Swiss or English flint glass, with a double convex of 
plate, and has been made achromatic by the form given to the outer curve of 
the convex, the proportion has been such between the refractive and dispersive 
powers of its lenses, that its figure has been correct for rays issuing from some 
point in its axis not far from its principal focus on its plane side, and either 
tending to a conjugate focus within the tube of a microscope, or emerging 
nearly parallel. 
Let a b be supposed such an object-glass, and let it be roughly 
considered as a plano-convex lens, with a curve a c b running 
through it, at which the spherical and chromatic errors are 
corrected, which are generated at the two outer surfaces ; and 
let the glass be thus free from aberration for rays f d e g 
issuing from the radiant point f\ h e being a perpendicular to 
the convex surface, and i d to the plane one. Under these cir- 
cumstances the angle of emergence g eh much exceeds that of 
incidence f d i, being probably almost three times as great. 
If the radiant is now made to approach the glass so that the 
course of the ray fdeg shall be more divergent from the 
axis, as the angles of incidence and emergence become more 
nearly equal to each other, the spherical aberration produced 
by the two will be found to bear a less proportion to the op- 
posing error of the single correcting curve a c b ; for such a 
focus therefore the rays will be over-corrected. 
