132 BACTERIOLOGY. 



erate aperture. The high objective, with a wide aperture, 

 is to be used only as the special occasion requires. 



In England and America, it is the custom to designate objectives 

 by their equivalent focal distance. On the Continent, however, no 

 such designation exists and some makers mark the objectives with 

 numerals, others with letters. Leitz, for example, marks his objec- 

 tives with numbers from 1, which corresponds to a If inch, to 10 which 

 is a T V water immersion objective. Zeiss, on the other hand, desig- 

 nates the achromatic objectives with letters, while the apochromatics 

 are designated by the equivalent focal distance expressed in mm. The 

 oil immersion lenses are designated after the English method T V, fa 

 iV inch. The objectives which have met with most favor in bacter- 

 iological studies are the 3, 7 and fa The No. 3 corresponds to a t 

 inch, and to Zeiss' A. The No. 7 is about the same as a i inch, while 

 the Zeiss D is a inch. 



The eye-piece, or ocular, serves to magnify the image 

 made by the objective. The form commonly employed, is 

 that known as the Huyghenian. It consists of two lenses, 

 a lower or field-glass, and an upper or eye-glass. Both 

 lenses are plano-convex. The lower, broad lens serves to 

 refract the rays of light, thus bringing the image within 

 the focus of the eye-glass. 



When apochromatic objectives are employed, it is nec- 

 essary to use special oculars, known as the compensa- 

 tion eye-pieces. The eye-piece represented, in section, in 

 Fig. 17, p. 125 is of this type. 



The designation of eye-pieces is subject to variation, as 

 in the case of objectives. Some designate by letters, others 

 by numbers. In this country, they are designated in the 

 same way as the objectives, that is, according to their 

 equivalent focal distance. Thus, a 2-inch eye-piece would 

 magnify the same as a single lens of 2-inch focus. The eye 

 pieces of low power are spoken of as " shallow," whil 

 those of high power are called " deep." The compensating 

 eye-pieces of Zeiss are designated by their amplification. 

 Thus, a 2 eye-piece doubles the initial magnification of the 

 objective; an 18 eye-piece magnifies the image 18 times. 



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