54 MICROSCOPY. 
uniform nomenclature of the value of achromatic objectives for 
the microscope, to which the foremost microscopists of our coun- 
try and abroad have advanced their contributions. 
The problem is a complicated one, and the following will by no 
Fig. 7. 
E 
A 
means diminish the practical difficulties, but will only add one 
more which has not been brought into consideration, 
Undue importance is given to the optical centre of a lens, or 
combination of lenses, by the different writers upon the subject, 
Fig. 8. while the great importance 
of the conjugate centres of a 
lens has been entirely neg- 
lected. The conjugate foci 
of a lens or combination of 
lenses, are in no way depend- 
ent on its optical centre, but 
- entirely on the conjugate cen- 
tres. The single plano-con- — 
vex lens makes an exception : 
for in this the optical centre 
and the conjugate centres 
fall together, where the opti- 
cal axis meets the curved 
surface. 
i If we take, for instance, a double convex lens of equal radii, 
= Fig. ' 7, its tape centre is O, and consequently the rays A’ and A, 
| in ns at such angles as to pass through the optical 
Senin : emerge at E’ and E, parallel with the first directions. 
1P mow the rays A At and A are y eaol towards the optical axis 
