58 Modern Microscopy 



Some of the best of the achromatic objectives, to which 

 reference has already been made, will stand as high an 

 eyepiece power as the apochromats, but generally they do 

 not advantageously bear anything higher than, say up to 

 10 or 12 diameters, and usually not in the perfect manner 

 that the apochromats do. The Huyghenian eyepieces that 

 are used with the achromatics are very rarely designed to 

 work one after the other in the same focal plane, with the 

 result that it is necessary to ref ocus every time the eyepieces 

 are exchanged, and the higher the power of the eyepiece 

 that is employed the closer will the objective work to 

 the object. The special convenience derivable with the 

 apochromats is, therefore, practically non-existent with the 

 achromatic objectives. 



Magnifying power, however, is not the only feature to 

 be considered with regard to an objective ; there must 

 be a power of delineating fine detail. This latter quality 

 is dependent on the numerical aperture of the objective, 

 referred to on page 60. 



IMMERSION OBJECTIVES. In using these objectives a film 

 of a specified fluid is interposed between the front lens of the 

 objective and the cover-glass of the object under examina- 

 tion, so that continuity is established between them. There 

 are two media that are in regular use, viz., water and cedar- 

 wood oil. Others, including glycerine and mono-bromide of 

 naphthalin, are, however, occasionally employed. It may be 

 taken that when a lens is referred to as a ' homogeneous or 

 immersion ' objective, that cedar-wood oil, or a mixture, of 

 which that oil is the principal ingredient, known as immer- 

 sion oil, is the correct medium for using with that objec- 

 tive. The refractive index of cedar-wood oil is about 

 1*52, and practically the same as crown-glass, consequently, 

 when it is used for immersion purposes it has the effect 

 of rendering the cover-glass part of the objective. 



The question naturally arises, What advantage is gained 

 by the use of an immersion medium ? In reply, it may 

 be briefly stated that the resolving power of the objective, 



