Ch. I] OBJECTIVES FOR THE MICROSCOPE 19 



Objectives and their Designation 

 § 19. Equivalent focus. — In America, England, and now also 

 on the Continent, objectives are designated by their equivalent focal 

 length. This length is given either in inches (usually contracted to 

 in.) or in millimeters (mm.). Thus: An objective designated xV in. 

 or 2 mm. indicates that the objective produces a real image of the 

 same size as is produced by a simple converging lens whose principal 

 focal distance is yV inch or 2 millimeters (fig. 10). An objective 

 marked 3 in. or 75 mm. produces approximately the same sized real 

 image as a simple converging lens of 3 inches or 75 miUimeters focal 

 length. 



As in microscopic work the object is always very close to the prin- 

 cipal focal plane, the magnification of the image is very approximately 

 the number obtained by dividing the image distance (fig. 84) by the 

 equivalent focus of the objective. It follows from this that the less 

 the focal distance of the objective, the greater is the size of the real 

 image, as the image distance remains constant. For example, if the 

 image distance is 250 mm., the real image of a 2 mm. objective is ^f-, 

 or 125 times longer than the object: of a 5 mm. objective it is ^|^= 50 

 times longer, etc., i.e., in these examples the magnification is 125 and 

 50 respectively (§ 19a). 



§ 19a. Initial magnification. — In addition to the equivalent focus, some 

 modern objectives are marked with their so-called initial magnification. By 

 this is meant the magnifying power of the objective alone at some standard 

 image distance. For example, the initial magnification of the Zeiss 2 mm. 

 apochromatic objective is given as 125. That is, the image distance is taken 

 as 250 mm. (^|^ = 125). With some opticians (Spencer Le,ns Co., Bausch 

 & Lomb Optical Co.) the initial magnification is that number which multi- 

 plied by the power of the ocular gives the final magnification of the entire micro- 

 scope (tube-length 160 mm., projection distance of the virtual image by the 

 eye, 250 mm.)- If one multiplies the initial magnification by the equivalent 

 focus in a list of these objectives, the image distance of the real image of the 

 objective will be found to vary from 160 to 180 mm. That is, the image 

 distance divided by the equivalent focus would give the initial magnification 

 listed, only by varying the image distance. 



§ 20. Numbering or lettering objectives. — Instead of designating 

 objectives by their equivalent focus, many Continental opticians 

 use letters or figures for this -purpose; in most cases, however, the 

 equivalent focus is also given. With this method the smaller the 



