18 
THEORY OF THE MICROSCOPE. 
In the same manner it might be shown that the spherical 
aberration of the lateral pencils will be very small, because 
they have been so condensed by the action of the object-glass 
that a pencil of light from any point in the object, occupies 
but a very small space in either the field-lens or eye-lens. 
Examining the pencil from one extremity of the object, 
which, without the intervention of the field-lens, would have 
converged to R and B, we see that its central ray, represented 
by the smooth white line which passed through the optical cen- 
tre of the object-glass, arrives at the field-lens uncolored, but 
is there divided into blue and red rays, (and other colors not 
represented in the figure.) The blue ray, which is refracted 
more strongly than the red, falls nearer the centre of the eye- 
lenSy where its refractive power is small, while the red ray, 
which is feebly refracted, falls nearer the border of the eye- 
lens, where the curvature of the lens increases its refraction, so 
that it emerges from the eye-lens very nearly parallel with the 
blue ray from which it was separated by the field-lens. In the 
same manner all the red rays occupy the parts of the eye-lens 
where the refractive power of the lens is greater than at the 
points occupied by the corresponding blue rays. Thus the 
chromatic aberration of the field-lens is very nearly corrected 
by the eye-lens. This is the property of the negative eye-piece 
pointed out by Boscovich. The excess of chromatic aberration 
in the eye-lens is balanced by a small amount of over-correction 
in the object-glass. 
31. Use of tlie term Acliroiiiatic Objective. From what 
has been stated, it is obvious that we mean by an achromatic 
object-glass, or objective, one in which the usual order of disper- 
sion is so far reversed, that the light, after undergoing the singu- 
larl}^ beautiful series of changes effected by the eye-piece, shall 
come uncolored to the eye. IIo specific rules can be given for 
producing these results. Close study of the formulae for achro- 
matism, and accurate calculation of the due proportions of all 
the parts of the instrument, are essential, but the principles 
must be brought to the test of repeated experiment. Nor will 
the experiments be of any value, unless the curves be most ac- 
J. & W. GRUNOW & GO’S ILLUSTRATED 
