20 



PEACTICAL PHOTO-MICEOGEAPHY 



therefore, that with an oil-immersion objective more light enters, 

 and light also of a very much wider angle, than is the case with 

 a dry lens. 



The difference between achromatic and apochromatic 

 objectives is a fundamental one, although it must be admitted 

 that the dividing-line is becoming ever finer. It is unfortunately 

 the fact that some lenses described and sold as apochromatic 

 are not exactly entitled to be so designated. Eecently the 

 term ' semi-apochromatic ' has been introduced, and is now 



FIG. 6. Diagram showing Effect of Oil Immersion as compared with 

 Dry Lens. 



frequently used to describe improved achromatic lenses, specially 

 those in which fluorite is used as one of the components of the 

 optical system. This term would appear to be a somewhat 

 unfortunate one, as although the lenses are an improvement 

 on the ordinary achromatic type, yet they do not compare 

 with the apochromatic lenses in perfection of correction. It 

 is unquestionable that achromatic lenses have of late been 

 greatly improved, and that their corrections are becoming ever 

 more perfect, but they are not yet entitled to rank with the 

 apochromats. 



The difference between these two types of lenses is princi- 



