Principles of Microscopy 13 



necessary to convert 2 mm to y 12 in. before arriving at Xl20. Actually- 

 most so-called "2-mm" lenses have a primary magnification of X90, 

 which reduces the whole system to imbecility. 



Working Distance. The working distance of an objective is the actual 

 distance between the front surface of the lens and the coverslip. This 

 has no relation to the equivalent focus since it is just as dependent on 

 N.A. as on magnification. Reference back to Figs. 9 and 10 will make 

 this clear. It is impossible to increase the theoretical N.A. of a lens 

 without widening the angle of the entering cone of light. It is impossible 

 to increase the angle of the cone without pushing the lens closer to the 

 coverslip. The working distance of a lens of large N.A. is therefore very 

 short no matter what the actual N.A. at which the system is operating. 

 This is not of much importance with low-power lenses but is critical 

 with high-power lenses such as the "high-dry" X45 (the old 4 mm). 

 Many student microscopes of the type shown in Figs. 9 and 18 are fur- 

 nished with high-dry lenses of N.A. 0.85. These lenses cannot possibly 

 be used at an N.A. greater than 0.6, so the higher N.A. has no purpose 

 except to ensure a large number of broken coverslips at the hands of a 

 beginning student. 



Working distance also decreases as actual image magnification in- 

 creases. It is almost impossible, for example, to use a X45, N.A. 0.95 

 objective on a microprojector since the huge magnification of the pro- 

 jected image requires a working distance less than the thickness of an 

 average coverslip. 



Working distance, in short, is a very important practical factor in the 

 selection of objectives. Very expensive lenses of high magnification and 

 large N.A. may occasionally be necessary in research, but they have no 

 place in teaching or routine laboratory work. 



Correction of Objectives. The methods by which corrections of lenses 

 are made have been discussed earlier. The compromises mentioned there 

 have resulted in two series of lenses known as "achromats" and "apochro- 

 mats." The words mean, respectively, "without color" and "with separated 

 colors." 



A theoretically perfect achromat, which does not exist, would bring all 

 wavelengths of light to a common focus so that, in terms of Fig. 4, all 

 the images would be of the same size. This is not practically possible, so 

 an endeavor is made to bring together those colors— red and green— to 

 which the eye is particularly sensitive. The residual blue color fringe of 

 the image is not readily detected by the eye and vanishes completely 

 against a blue background. For this reason, manufacturers commonly 

 provide a disc of blue glass in front of cheap microscope lamps. Green 

 glass has exactly the same effect and is far more restful to the eye. 



Achromatic lenses are relatively cheap and are adequate for most 



