i6 



ELEMENTS Oh 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 Abbe 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 



Fig. 14. — Epfect 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 



