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 F2; Oi, object; O2, real image in F2, trans¬ 
posed by the collective lens, to O3, real image in eyepiece diaphragm; O4, virtual 
image formed at the projection distance C, 250 mm. from EP, eyepoint; CD, 
condenser diaphragm; L, mechanical tube length (160 mm.); 1, 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.) 
