86 



MICROSCOPY 



part of the body tube to be rotated. The eyepiece can point backward, 

 more nearly hke the conventional microscope, or forward as shown. In 

 many of the new models, the prism system also divides the light between 



two ocular lenses so that both eyes 

 can be used in viewing. Focusing 

 is accomplished by raising and low- 

 ering the stage rather than the body 

 tube. This arrangement is advan- 

 tW 'M^WK N. tageous because the eye and head 



\^Sp>j ^^^^ \ need not move upward and down- 



ward, but it may make the control 

 of illumination from below slightly 

 more difficult. Several of the newer 

 models even incorporate a third 

 ocular tube to which a camera is 

 attached. The object can thus be 

 located and examined and then 

 photographed very conveniently. 



The object might be illuminated 

 with daylight from a window by 

 means of a substage mirror, but now- 

 adays artificial light is used more 

 commonly. Light from a lamp can 

 be directed toward the mirror or a 

 small lamp can be placed beneath the condenser in place of the mirror 

 Figure 8-3 is a diagram of image formation in a compound micro- 

 scope. The objective lens produces an image of the object at the level 

 of the eyepiece. Further magnification occurs when the eyepiece, with 

 the help of the lens of the eye itself, focuses the image on the retina. 

 The eye interprets what it sees as if the ocular lens produced an enlarged 

 image, or virtual image, at a distance of about 250 mm from the eye. 

 Probably the ability to imagine the image at that distance requires some 

 practice initially, but it comes quite naturally to anyone with a little 

 experience. 



Fig, 8-2. A recent model, high-grade 

 microscope. (Courtesy E. Leitz, Inc.) 



Optical theory 



All optical instruments, including the eye, depend upon light. Light 

 moves as vibratory energy in straight lines outward from a source. In 



