80 PHOTOMICROGRAPHY 



these lenses is taken, and the N.A. calculated, the latter 

 will be found to be small. A lens with an aperture given 

 by f /4-5, and used to obtain a magnification of 4 diameters, 



is working with a N.A. given by N.A.=- 77XT\ = '09. 



This will indicate that no high degree of resolution is to 

 be expected, but for the purposes for which these lenses 

 are intended this is of small account, their flatness of 

 field is a much more useful property. 



These lenses are fitted with an iris diaphragm, which 

 is used to close down the aperture when greater depth of 

 focus is desired. If the iris is not placed in quite the 

 correct position in the lens, it will restrict the field 

 of view as it is closed down. They will, in some cases, 

 cover an object whose diameter is as much as their 

 focal length. 



The magnification they give under various conditions 

 of camera extension can be calculated from the formula 



- 



Where M= magnification. 



E =distance of focussing screen from diaphragm 



of lens. 

 F = focal length. 



It is interesting to note that some of these objectives 

 can be used on small pocket cameras, so giving them 

 a double usefulness. If the lens is unsymmetrical, it 

 should be placed in the camera with the lens which 

 faces the object in photomicrography turned towards 

 the focussing screen. 



Sets of Objectives and Eyepieces. A Complete Set of 

 Objectives comprises the following lenses given in tabular 

 form with approximate magnifications measured under 

 the conditions described above. 



