Microscope Construction, Use, and Care 183 



and magnified, and improved in quality by an eyepiece. 1 he eyepiece 

 or ocular consists ot two or more lenses working as a unit and having 

 a fixed magnification. It a ground-glass screen, a sheet ol paper, or 

 a photograj:)hic plate is placed at any plane above the eyepoint ot 

 the ocular, an image is produced (Fig. 16.1 B) . Note that the primary 

 image is inverted and the projected image is erect. 



Objecf 



Lens 



Image 



^ 



D 



Object 



Objecilve 



Primary 

 image 



Ocular 



Projecfed 

 ■° imaqe 



Fig. 16.1— Formation of projected images by the microscope: A, simple microscope; 



B, comjjountl microscope.^ 



With a given objective and ocular, the size of the image varies 

 with the distance of the screen from the ocular. If the screen is placed 

 approximately 10 in. (254 mm.) from the eyepoint, the size of the 

 image will be approximately equal to the product of the designated 

 magnifications of the objective and ocular. Thus, an objective having 

 a designated magnification of lOX. used with a lOx ocular, gives 

 a total magnification of approximately lOOX- Exact values must be 

 determined by micrometry. 



The foregoing discussion does not take into account the operation 

 of the human eye working in conjunction with the microscope. 

 However, most microscopic work is done by chrect visual observation 

 with the eye held at the eyepoint of the ocular. Let us turn for a 

 moment to a consideration of the eye as an optical instrument. The 

 lens of the eye operates as a simple lens, and the curved retina is 



1 The illustrations in this chapter are highly diagrammatic and simplified and are in- 

 tended only to show the approximate relative positions of the object, the optical elements, 

 and the images. 



