FUNDAMENTAL CONSIDERATIONS 



II 



FIG. 4. Diagram illustrating optics of a compound microscope in use. Fi, 

 Upper focal plane of objective; F2, Lower focal plane of eyepiece; A, Optical tube 

 length = distance between Fi and Fa; Oi, object; Oz, real image in F2, trans- 

 posed by the collective lens, to O 3 , real image in eyepiece diaphragm; O-t, virtual 

 image formed at the projection distance C, 250 mm. from EP, eyepoint; CD, 

 condenser diaphragm; L, mechanical tube length (160 mm.); i, 2, 3, three pencils 

 of parallel light coming from different points of a distant illuminant, for instance, 

 a white cloud, which illuminate three different points of the object. (Courtesy 

 of Bausch and Lomb Optical Co.) 



