MICROSCOPY. 55 
of the lens, they meet at a point, C’, the centre of admission. If 
the rays E’ and E are prolonged, they will meet at C, the centre of 
emission. Therefore the conjugate foci do not meet at the optical 
centre O, but are to be measured from C’ to the object, and from C 
to the image; and the sum of the conjugate foci is not equal to 
the distance between object and image, but in this case the dis- 
tance between C and C’ must be deducted. 
In combinations of lenses it is precisely the same. It is almost 
impossible to analyze such a complicated system as a modern 
microscopical objective, and to fix the position of the optical 
centre or the conjugate centres, although all combinations possess 
these remarkable centres. But let us take a simple combination of 
two plano-convex lenses placed symmetrically, in which it is not 
difficult to determine all that we need. In such a combination, 
Fig. 8, the rays A and A’ pass through the optical centre O, and 
emerge to E and E’, parallel with their original directions. Now 
if we prolong A and A’, they will meet at C’, the centre of admis- 
sion; and E and E’ prolonged will meet at C, the centre of 
emission. To find for this combination the relation of conjugate 
foci, or the relation between the size of object and image, we have 
to compare the triangle ECE’, with the triangle A’C’A. In this 
case the sum of the conjugate foci is equal to the distance of 
object and image, plus the distance from C’ to C. In combina- 
tions this will generally be the case. — JOSEPH ZENTMAYER, Phila- 
delphia, Sept. 25th, 1872. 
AmputrLevra Perivcrpa By Moontieur. — Many microscopists 
have had the curiosity to use the beautiful white light of the full 
moon as a source of microscopical illumination, but probably few 
have tried it upon the more difficult objects. Prof. T. D. Biscoe, 
led on by the clear sharp view given by it of easier objects, tested 
it upon the last diatom of the Test Plate, using a Hartnack objec- 
tive No. 10, and resolved the “test” at first trial. 
Tur Stupy or Licuens.—The explanation of the peculiar double 
nature of the lichens has lately become the subject of much dis- 
cussion. It has been long recognized that in the tissue of lichens, 
are to be found two quite distinct classes of elements. By one 
class the lichens are allied with the fungi, by the other with the 
. The great body of a lichen is made up of a structure 
exactly identical with certain fungi, wliile scattered through the 
