16 ELEMENTS OF APPLIED MICROSCOPY. 
site surfaces of the lenses according to certain known 
relations. Such lenses are designated as aplanatic. 
There still remained one serious limitation to the in- 
crease of the power of magnification of the compound 
microscope. In all its various refractions there is a loss 
of light, and with the small objective lenses of great curva- 
ture, necessary for high magnification, it is difficult to get 
a sufficient illumination for clear vision. Furthermore, 
it has been shown by Abbé and others that the rays 
which extend from a point toward the periphery of a 
lens are of prime importance in the detection of 
the finer structure of objects. If the outer zones of 
rays are lost, no image of very minute details can 
be formed; and this is just what occurs in the ordi- 
nary compound microscope when there is air between 
the objective and the specimen to be examined, as is 
shown in the right half of Fig. 14. If, on the other 
Fic. 14.—Errect or HoMoGENEOUS IMMERSION, (After Hager-Mez.) 
hand, some substance like cedar-oil, which has the same 
refractive index as glass, be placed between the lens and 
the cover-slip which covers the specimen, the rays will 
