1894 THE MICROSCOPE. 133 



Immersion Objectives. 



By A. C STOKES, M.D., 



TRENTON, N, J. 



From his new book "Microscopical Praxis." 



Immersion-objectives are those that require a drop of 

 liquid between the front lens and the cover-glass when 

 in use. Among the advantages obtainable by their em- 

 ployment are increase of working-distance and increase 

 of numerical aperture, with, as a consequence, the ad- 

 mission of more light, and especially the partial, or in 

 some cases, the complete extinguishment of the cover- 

 glass, by which its aberrations are obviated and two re- 

 flecting surfaces, those of the cover and of the front 

 lens, are cancelled. The honor of discovering the prin- 

 ciple is usually conceded to the renowned Italian pro- 

 fessor, G. B. Amici, who exhibited water-immersion ob- 

 jectives at Paris in 1855. It is also said that he used 

 oil as well as water for the immersion-medium, and that 

 he therefore deserves the credit for originating oil-im- 

 mersion objectives. For the modern homogeneous im- 

 mersion, however, we are indebted to Mr. J. W. Steven- 

 son, a well-known British microscopist. It was he who 

 suggested to Professor Abbe and to Dr. Zeiss that they 

 should turn their attention to the theoretical and the 

 practical application of the principle ; it is to him, there- 

 fore, that we owe this great modern advance in practi- 

 cal microscopy. 



The use of immersion-objectives is attended by a lit- 

 tle more inconvenience than that of dry objectives, and 

 the immersion fluid is likely to be carried over the edge 

 of the cover by the movements of the stage, and so is 

 liable to mingle with the mounting medium in those 

 preparations which are not permanently sealed. This 

 annoyance may be avoided to a great extent by using 

 square covers somewhat larger than the cement ring en- 



