THE YOUNG SCIENTIST. 
in the degree requisite for balancing the 
under-correction of the single middle and 
front when set at the prescribed distance 
of the aplanatic focus, yet by trial on the 
mercury globule all the results invariably 
displayed two separated color-rings ; these 
could not be combined by alteration in the 
radius of the lenses. By projecting the 
blue or red, or visible rays of greatest 
and least refrangibility through the 
system, the cause became apparent. The 
left hand section of this object-glass is 
shown in Fig. 19. The rays from the focus 
are slightly divided by the first front sur- 
face. On enierging from the back the 
separation is increased ; the red ray (r) is 
outwards, and the more refrangible or 
blue ray (5) inwards. Next the diver- 
gence of these two rays is extended by the 
middle single lens. The following crown 
lens extends the angle of divergence so 
far that the flint lens of the back triple 
cannot recombine them ; and they emerge 
at two distinct zones, shown by the prac- 
tical test of the " artificial star " or light- 
spot reflected from a mercury globule, 
. viewed within and without the focus. 
It might be supposed that these rays at 
their final emergence can be so refracted 
as to project the blue outwards. A cross- 
point would then occur at a fixed conju- 
gate focus in the body of the microscope, 
at which all rays would be combined, and 
if this focus was adjusted to that of the 
eye-piece, achromatism and final correc- 
tion would be the result. But to meet 
the various conditions occuring in the use 
of the microscope, the conjugate focus 
constantly alters in position, this being 
affected by every change of eye-piece, 
length of tube, or adjustment for thick- 
ness of cover ; therefore a correction for 
a fixed point cannot be maintained. 
Achromatism in the microscope object- 
glass, like that of other perfectly corrected 
optical combinations, must be the re- 
union of the rays of the spectrum close to 
the final emergent surface of the system. 
The remedy suggested by these experi- 
ments appeared to be in a transposition, 
that is, in placing the over-corrected 
triple in the middle of the entire object- 
glass ; this would at once cause a conver- 
gence of the blue and red rays. A single 
lens of longer focus at the back would 
then bring these rays parallel at the point ' 
of final emergence. 
By projection in a diagram, this con- | 
dition was apparently realized. The dis- \ 
persive power of, the flint (density 3*686) 
was taken by the refractive index 176 of 
line H in the blue ray of the spectrum, 
and 170 of the line B in the red ray. The 
refraction of the corresponding rays in | 
the crown (density 2*44) was 1*53 H and | 
1'51 B. With these indices the rays are j 
traced in Fig. 19. The radii in the right- 1 
hand half section are these of an "eighth" 
of the new form drawn about 15 times the 
size of the original. The single front i& 
of the usual form, as this is much alike in 
all cases. The radius or focus of the 
single plano-convex back is about four | 
and a half times that of the front, and 
the focus of the middle (triple) three | 
times. The passage of the blue and red i 
rays at the extreme of the pencil is ^ 
shown in contrast with the preceding, 
the separation from the same front being 
alike. i 
The inner and outer, or blue and red 
rays, after passing the first surface of the 
triple middle, meet the concaves of the 
flint, which refract the blue rays to a 
greater extent than the red, and cause 
them to converge (instead of diverging, as 
in the opposing half diagram) so that at 
their exit from the triple they meet and 
would cross, effecting what is known as 
''over-correction ; " but this is so balanced 
and readjusted by the single back of 
crown-glass that the rays, are finally | 
united, and emerge in a state of paral-' I 
lelism. This form of object-glass is suit- | 
able for the high powers, or such as have 
a cover adjustment, viz., from the " k- 
inch" upwards; perfect color-correction i 
is equally to be obtained in all of them. i 
It may be asked by some who have i 
devoted their attention to the higher 
branches of optical mathematics, why | 
the above result should have been worked 
out entirely by diagrams. But it has been 
found such a difficult task to calculate the ! 
passage of the two rays of greatest and I 
least refrangibility through a combina- 1 
tion having sixteen surfaces of glass of I 
three different densities and retractions,. 
