34 THE ELEMENTS OF VEGETABLE HISTOLOGY 



FIG. 18. Diagram Showing Path of Light Rays. 



Fi, Upper focal plane of objective. F, Lower focal plane of eyepiece. A, Optical 

 tube length ^distance between Fi and F^. O\, Object. Oz, Real image in Fi trans- 

 posed by the collective lens, to Os, Real image in eyepiece diaphragm. Ot, Virtual 

 image formed at the projection distance C, 250 mm. from EP, Eyepoint. CD, Con- 

 denser 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. 



